With flat base boards, I need to produce some high ground, not true mountain scale hills. More like rolling soft hills. Not to block movement on the table, more to provide cover and block lines if sight. The main thing that has made this work for me is a great product called mod rock in the UK. Its gauze bandage impregnated with plaster. Think of the old style broken leg style plaster casts, that's the stuff. Got mine from Hobby craft for about £5 for a roll. As used by sculptures.
On the left of the picture you can see the art board base, two CD's in size, and with the skeleton structure of foam board hot glued into position. To the right is a 3 CD size base with the mod rock on. I've found that to avoid the art board base becoming to wet and warping when dried out. Its best to cut the Mod Rock into lengths of about 4 -5 inches, dip into a bowl of water, then quickly squeeze out most of the water, so that when you apply the mod rock to the card skeleton its not over saturated with water.
Once the Mod Rock has dried out you will still be able to see some of the gauze showing, so I go over it with a thin layer of filler and then PVA and sand it, and add in some small rocks to Finnish it of Double click on pictures for bigger version