I learned in my psychology classes that memory is most deeply encoded when associated with concrete imagery. That is, memories are strongest when they appeal to all the senses and can readily recreate an imaginable scene or context. Rote memorization (e.g. reviewing flashcards over and over) is considered a weak encoding technique because it relies solely on repetition to encode memory. Herein lies the problem with remembering new vocabulary, especially new vocabulary in a second language. Is there some way to study it using "stronger" memory encoding techniques? Rote flashcards are boring and don't work very well. The memory of them is literally gone in a flash.
Mnemonic devices are shortcuts used to memorize information that is tedious and difficult to learn using rote memorization. Some well-known examples include "Every Good Boy Does Fine" and "All Cows Eat Grass," mnemonic devices used to memorize sheet music notes. These two sentences connect poorly-imagined letters (ACEG) to well-imagined scenes (a cow eating grass). It only takes a few rehearsals to remember them, and they stick far better than repeating the letters over and over again. Mnemonic devices are efficient and powerful learning tools.
I use them when studying Japanese vocabulary by creating and adding them to my flashcards. Here are some examples:
守る（まもる）- to protect
The MA protects her children using a lawn MOWER.
"Protect" in Japanese is "mamoru." The above mnemonic device uses only English words and paints a vivid and bizarre scene making the word difficult to forget.
The ducks decided to compromise today.
"Compromise" in Japanese is "dakyou." The first part sounds like the English word "duck," and the second like the Japanese word for today (今日). This mnemonic device utilizes imagery connected to both English and Japanese words.
専門（せんもん）- professional / specialist
A thousand gates open for professionals.
"Professional" in Japanese is "senmon." The first syllable shares its pronunciation with the Japanese word for 1000, while the second syllable literally means "gate." This mnemonic device uses only previously-learned Japanese words.
I majored in psychology, so I know what you're thinking. You're thinking, "You gotta be kidding me. How can I come up with mnemonic devices for the thousands of words that I need to study?" First, like anything new it requires practice, and with practice it gets easier. Second, an effective mnemonic device teaches you the word once and for all. It may take time to create a good mnemonic, but once you do you're done. Albeit challenging, it's time well-spent. Last, the mnemonic devices don't have to be bizarre images. They can be based on people or places you've encountered, or simply reminders of other contexts in which you have met the word before. Consider them study notes to help you remember. Some other examples:
個性（こせい）- individuality / personality
Think of your friend KO-hei and all the crazy stuff he SAYs. He has quite a personality.
I based this one on a Japanese guy I once met. Lucky for me he had quite a colorful personality and his name made it conducive for creating this mnemonic.
Think of the lyrics in the song "Sakura Saku" - 手を延ばして！
I based this one on the lyrics of a cheesy anime song that sometimes gets stuck in my head.
As musicians study sheet music they reach a point where they no longer need "every good boy does fine." They've so deeply encoded the memory that it's become automatic. This also happens when studying vocabulary. As I study new words, I begin to hear and recognize them in multiple contexts. Later I can recall them, use them in conversation, and generate sentences using them. Encountering and using the words in multiple real contexts further deepens the memory encoding to the point where I no longer rely upon the mnemonic to remember it. The training wheels on that word come off, then I move on to create more training wheels for more words. It's worked well for me, so maybe it will work for you.