Wayne,
Shamus's method is the best way to replicate rolling scenery. I've always started with a flat world and worked from there. Ranges are easy, it's that ravine you wanted that's hard to make if you don't do it Shamus' way!
Shamus also brings up a good point in using seasoned wood, and let me take it one step further. Don't buy the wood when there's $$$ and no time. Buy when you're ready to work and put it together.
I purchased a small load that ended up in the garage for several months when project time was diverted to other things. When the cheapo developer built my house, instead of venting the clothes dryer outside, he put the vent inside the garage! I suppose this was a poor-man's way of heating an unheated garage instead of venting the hot, moist air outside. Even though the wood was seasoned, it warped to uselessness. This event made me re-evaluate what I stored in the garage, and I learned the hard way only buy wood when I was ready to work with it.
My deck is 1/2" plywood sheets topped with 1/2" sheets of Homasote. Now we've had a lot of screams here, and in other sites that Homasote isn't readily available in most parts of the country. But if you can get it, I swear by it. It doesn't warp, and it's great for driving track nails into with ease and it holds up great. When working with moist/wet scenery materials, it absorbs and dries without repercussions.
One last thing that comes to mind, if you're doing an "island" type of layout, I gave myself four feet of clearance from the layout edge to the wall. This way, people can get by each other without incident and you can get back far enough to enjoy the view yourself. I would have prefered an around the wall type, but the room I have this time prohibits it. I've tried two and three foot clearances in the past and they make life miserable.
What size are you working with, and are you going for an island or around the wall?
George.