It's time to shift your focus from just 'learning words' to building a lexicon. Luckily, this is something you can work on from the very first Japanese word you ever learn, and you can start at any time.
These techniques will help you not only learn new works easily, but retain them forever, just like you do with words in your native language.
A main point here is that you're going to be learning words quite similarly to the way you learn new words in your native language. That means you're not going to be learning them through translation. For more details on what I mean by 'learning through translation' and why it severely hurts your Japanese (and how to stop doing it to improve your Japanese!), read this article about the biggest mistake you can make in learning Japanese.
Whenever you discover a new word in Japanese that you want to commit to memory, follow these steps. (In fact, you can and should do this with new English words, too, to help you have a good native vocabulary!)
Note that this doesn't involve any instructions for learning to write words. It's normal to know how to say more than you can write, and it's normal to learn how to write something after learning how to say and hear it. This is even true for your native language. You don't have to learn how to write a word to learn it. You can always learn to write it later when you're more comfortable. In fact, I find learning how to write words I already know is easier than trying to learn to write a word when I'm still learning it verbally.
1. Find a basic definitionThe first thing you should do when you see a Japanese word you don't know is find a definition. Usually you can do this through a dictionary. If you're using a textbook you probably already have this. If you're speaking Japanese with someone, just ask them what it means. If you both are speaking Japanese, try to have them explain it to you in Japanese, not just give you a translation.
Or if you're wondering how to say a certain thing in Japanese, you can look it up through English-Japanese dictionaries or ask someone who speaks Japanese. Once you get your Japanese, word, I suggest looking it up again using Japanese-English, or in a normal Japanese native dictionary if you know enough to use one.
If you are near a computer, by far the best resource you have is Jim Breen's WWWJDIC. WWWJDIC is an open online Japanese-English dictionary with a ton of extra functionality. Be sure to save a list of the mirrors or bookmark all the mirrors in case the main page is down.
Or if you know enough Japanese to understand a Japanese dictionary, I usually use dictionary.goo.ne.jp because it is the easiest for me to find.
Online dictionaries are great because you can copy/paste text, so it's easy to look up words you find online.
Anyway, now that you have a general idea of the English words associated with your Japanese word, you can go ahead and stop thinking about the English word associated with this. You're now researching a new word -- pretend it has nothing to do with any English word you've ever heard of, even if it's a simple translation.
Make sure you take note of what kind of word (part of speech) the word is. WWWJDIC will tell you this. Often, there will be examples, as well, so you can click on the examples button (labeled [Ex]) or read any that might pop up that can easily show you how it's used.
2. Say the word. Like a lot.Before you even say the word the first time, look up the pitch accent pattern for it. This is important. If you have a Japanese dictionary or textbook that tells you the pitch accent, great. An easy way to look this up online is through OJAD, the Online Japanese Accent Dictionary. It seems to be down a lot, but it will often be back up in like 5 minutes so keep trying if you can't access it.
Make sure you get the proper accent down. Take note if the word is unaccented or if it's not, what mora the accent ends on. If you don't understand pitch accent or don't even know what I'm talking about, learn it now, at least at a basic level, before you try to learn any more Japanese.
Try to find native Japanese speakers saying the word. OJAD can help a lot hear, but they don't have all the words. Sometimes WWWJDIC will have audio, too. It's good to hear it by itself, but also in context, so if you can, listen to people using it in sentences, too.
If you can, record yourself saying it and play it back to yourself and compare it to a recording of a native speaker.
Keep saying the word over and over. If you can find some easy examples of it used in sentences, say those, too. Use it in your own sentences. You can continue doing this step forever, including for the rest of these exercises. Any words you stumble over saying you should practice saying regularly. Get your facial muscles used to the motions of the word, both on its own and with other words surrounding it.
3. Gather illustration of the word.Now it's time to develop the idea of this word. At this point, you should not even remember the English word or any English you used to get to this point. You should be thinking of it as a purely new Japanese word that doesn't relate to English at all.
If you are learning this word because you heard it somewhere, think about the context you originally heard it in. Think about whatever that was and what image it puts in your head now that you have a better idea of what the word means. Think of that image, and repeat the word to yourself, making sure you're associating it with these ideas.
If you're around a native speaker, ask them to describe the word to you again (no translations!) and pay attention to the ideas that form in your head while hearing their description. Repeat the word to yourself and associate it with these ideas.
If you have examples of the word being used, read or listen to them again and think about the pictures in your head. Repeat the word to yourself and associate it with these ideas.
Go to a search engine with image search (use a Japanese one or one that can return Japanese web results) and search for the word. What kind of images come up? This of course works better for certain types of words, but it's always worth a shot.
Think of all the things you associate with all these ideas, and keep repeating the word to yourself, making sure you're telling yourself that this word means ALL of this, that this little universe of ideas is contained in this one little word. When you think of this word, it should bring back those images, just like when you hear words in your native language, you think of ideas and images.
4. Find ways to use the wordUp to now, you've probably only spent a minute or two studying your word (or a little more if you made a recording and compared and really worked hard at pronunciation).
Now it's all about practice. You have to practice the word to get your mind used to accessing this part of your lexicon.
If you speak to others in Japanese a lot, try to find ways to bring up the word, or if it's a common enough one, just be sure to use it when you can.
I'm going to assume, though, that you don't know anyone who speaks Japanese and you're living in a place where everyone speaks English so you have no opportunity to speak Japanese with others.
I find the easiest way to practice words is to just talk to yourself in Japanese. Try not to just parrot the same sentences over and over. Just have a little discussion with yourself so that you will be saying it in a lot of different ways.
If you're expressing the word in a written way (like using it on your blog or in tweets or just writing letters or something) make sure you're reading your own writing aloud.
Don't just say it in your head -- you have to actually say it aloud. You need to associate the muscle movements (including moving your vocal chords!) with the lexical entry. So don't whisper or mumble to yourself -- make sure you're talking in a normal way.
5. Describe the word to someone else.If you know someone who is also learning Japanese or just likes listening to you talk, describe the word to them. If you don't know anyone or they're just not around right now, pretend there is someone there.
Here's the challenge though: you can't use any English translations for the word. Completely avoid using those words that you saw in the Japanese-English dictionary earlier (you should have put them out of your mind by now anyway!) and describe the word as a concept.
If you and the person you're talking to (even if you're talking to yourself) understand Japanese well enough, just go ahead and describe it in Japanese. If not, describe it in English while avoiding any "direct" translations.
If you're describing the word to someone else and they ask you "Oh, so does it mean (translation)?" try to avoid saying yes. You can say "Pretty much" or something, but I don't ever like to say that any Japanese word "means" any English word. Because honestly, it never does. Bonus points if you can think of ways in which the concept differs from the one you associate with the given English "translation!"
If you're able to describe a word to someone else without using translations and they get it, you've successfully learned the word. As long as you keep running into it regularly enough for a while, you'll never forget it.
And the more words you learn this way, you'll be able to pick up new words faster and more easily. Eventually you'll be able to pick them up just like you do with English or your native language.