I recently built a sawbuck following the instructions here:
diy sawbuck
I altered he design a little.
First, as many in this thread have suggested you want the middle legs closer to one side.
Second, I put the lower crosspieces near the ground. It seemed to make it more stabel. Also, I found with my old sawbuck that I would put my foot on the lower piece to keep it steady when cutting by hand. Finally, in the interest of stacking functions, I can fold it up, lay it on the ground, and use it to rip short pieces of plywood.
Speaking of stacking functions, I cut dimensional wood more than logs, so I cut out notches to hold 2x4s and 1x4s. This, I hope, will be easier for doing cross-cuts than sawhorses are.