Here's the how to on a sectional game board I made. It was initially intended for 15mm WWII using Two Hour Wargames NUTS! rules, but it has been used in many different games.
The playing surface utilizes some foam interlocking pads I picked up at Big Lots! for a few dollars. I've seen the technique used several times for making modular, portable and durable gaming surfaces.
There are a total of 9 pieces in the set. I wanted each piece to line up with every other piece, so a technique similar to this one at Terragenesis was used to lay out the roads. Once all the roads were marked, fine sand was glued down. When this had dried, each piece was spray painted a dark, flat brown.
The roads were drybrushed brown and tan, and then a mix of ground cover was glued into place.
The pieces are not as flexible as I had hoped, and there has been some chipping at the joins from the pieces being linked and unlinked. Other boards I have seen has used a layer of mastic to remedy this problem, giving the tiles great flexibility.
Also, the texture of the mat shows through ever so slightly. Even with adding two layers of ground cover they weren't completely covered. Again, a layer of mastic would've helped fill in the texture.