I guess the obvious and generic rule would be keep the bulk of your mass as low to the ground and as centered as possible. In 2000, team 38 had a big boxy robot, which when tucked in, was very solid, almost impossible to tip. Unfortunately, the big boxy part extended out to latch on to the pull up bar. It was hard not to pop wheelies driving with the boxy part out, which made it very difficult to collect balls.
In 2001 and 2002, we kept very low to the ground (both robots were well under 2' tall, while filling the whole 30x36" footprint horizontally), which of course, meant supreme stability against tipping.
For the stacking last year, we had a forklift. Problem was, the mechanism for the arms was very heavy, and had to go pretty high up to stack. Combine that with relatively big pneumatic treadless tires, and we bounced more than we turned. We got knocked over so many times that year, sometimes with
amusing results.
This year, we tried to keep our center of gravity as low as possible, and met with pretty good success. Through both the UTC regional and Nationals, we were never flipped. I think the low and even placement of our drive gearbox, and centralized position of our ball collector kept our tires firmly on the ground, despite the van door motor sitting high above driving our 2x ball grabber.