Techniques for learning and retaining characters
You can use a SRS (Spaced Repetition System) software, I personally use Anki to create a deck of study cards and I review them on my mobile phone. 

I also recommend Memrise.

2014 update:

I still use Anki, but combined with Google Images Search and Forvo: When I make new flashcards, I'll add the writing (hanzi &amp; pinyin), its pronunciation (from Forvo) and a picture of the character's meaning (from Google Images), that way all the card's content is in Chinese, and I avoid relying on translation. By the way, Anki now allows to synchronize media files (audio/images) between devices.

I've started using this new method few months ago, and it has worked very well for me (especially with nouns). Credits of this method goes to the "Fluent Forever" book.