If you are playing Single Player the easiest way to take down the walkers is to let the computer A.I. fly the speeder. The awkward camera makes navigating the AT-AT legs very hard.
Spawn as a pilot in Echo Base and jump into a speeder, immediately switch to the tow cable gunners seat and any A.I. an order to "hop in". Sooner or later one will spawn to take the pilot seat. Once you are airborne give the pilot (repeated if neccessary) the "all clear, move out" order and he will begin an attack run on the walkers. Pay attention using the radar what side of the snowspeeder the walker is coming up on and look as far over as you can in that direction. As soon as you see a walker leg enter your view fire the tow cable, if you miss the near leg you will hit the far side leg. The pilot can navigate the walker just fine. While he is wrapping up the AT-AT use your rear cannons to fire on the ground to protect yourself from spawning stormtroopers with launchers. You wont get them all but you'll get a few. I've succeeded with this many times and even had two snowspeeders wrapping up the same walker at once.
As soon as the walker is down, take over the pilot seat and fly your speeder back towards the Outpost and provide ground support for the turrets. As soon as the Outpost is clear, land at a repair droid to fix any damage you've taken to your speeder.
Rinse and repeat.