Tumblr has 500KB GIF limitation, here's some tutorial and tips to optimize your animation.
ETA : Tumblr has upgraded their limit to 1MB per January 2012.
Ultimate tumblr GIF Tutorial
Photoshop CS5/CS4 Extended/CS3 Extended
(this tutorial is made with Photoshop CS3 Extended, Windows Vista OS)
(
tutorial and tips under the cut )