I do the same thing. I watch as they fingersign super slow... but to get it I usually end up making the sing. The other problem I noticed is that a lot of sites have fingerspelling practice but it is just a pic of a letter, then a pic of the next... the letters don't flow like in real fingerspelling. I'm not sure how much practicing with pics will help.
Hopefully someone will have some ideas for where to practice fingerspelling.