For something really sturdy, here's an idea that I've done:
find some long landscaping spikes (check the gardening section of your local hardware store - they look like really big nails)
Get 2 2x4's the length of the table you want (6' or 8' - or do 2 of each for variable table size)
drill two holes large enough to slide the landscaping spikes through 18" in from each end, from one narrow edge to the other
build 2 sawhorses (using 2x4's) the width of the table you want (4') (the height of the sawhorses should be almost the height of the table you want, minus the width of a 2x4 and your boards)
drill two holes large enough to slide the landscaping spikes through 12" in from each end, just like the 2x4's above
get two large (2' x 3' or 2' x 4' depending on table size) pieces of 1/2" plywood or similar material and drill holes for the landscaping spikes in these (I also suggest making counter-sink holes in the tops, so the landscaping spikes will sit flush in the boards). The holes should be in positions matching the holes in the 2x4's, so 12" in from one edge and 18" in from the other edge).
Now assemble the hole structure, putting up the sawhorses, laying the 2x4's on top, putting the boards on these, then dropping the landscaping spikes down through the hole assembly. It is a very stable structure. (I then lay 1/8" MDF boards over top of these with various colors and textures for varied battlefields.)
(With creative placement of additional holes, you could also probably use 2' x 2' boards and additional landscaping spikes -though some of these may need cut down as they won't be going through two sets of 2x4's)