I also use mnemonic devices to remember not only words, but also number plates, film titles etc.

Another technique I use is to learn the etymology of the word. That works great since an obscure word might turn out to be an easy word to remember if you knew the origin. For example, "preposterous" means absurd. It was not easy for me to remember its meaning, until I learnt that it came from the latin words prae, "pre" + posterus, "coming behind". Thus it means something whose end is coming before the beginning. An advantage of deconstructing words like this is that your vocabulary will grow bigger because there are many words that share the same latin or greek (or arabic, spanish, french...) roots.

Another thing I use to learn new words, and that I also enjoy very much, is reading. I read a lot, just about anything. This is great not only for learning words, but also for learning grammar.

So, as you can see, it's better to use more than one way to learn a new language.