I always love playing as Rebs on Hoth.. because Rebs have the potential to ALWAYS win.
First off, Snowspeeders pwn. Hardcore. If 1-2 go after the walkers, the rest can strafe enemy troops and turrets and generally screw things up.
I usually spawn as a pilot in the snowspeeder hangar. I typically wait till I see a bot spawn, order it to follow me, then I jump in a speeder (and the bot jumps in the back). Then what I do is fly towards the nearest AT-AT, let the bot spear the leg, and then I fly around and around till it drops
Rinse, repeat, and strafe when no walkers are available to kill
Snowspeeder blasters do minimal damage to walker vehicles (including AT-STs). I've found that if an AT-ST is already smoking, I can usually finish it off... otherwise it's pretty pointless to shoot it.
Of course, that may be because there was a pilot driving those AT-STs most of the time... but I donno :P