Personally I've found that the combination of skritter and Popup chinese works very well. I use Popup to learn to understand chinese and Skritter for learning how to write and for the endless repetition of characters.

I can't advise using only skritter though, since chances are high that you'll be frothing mad within a week or two of doing nothing but writing. Popup adds some much needed relief from that and more importantly, teaches how to use the words in context.

-- Noah