Today's lunchtime mapping tip - my workflow on mapping hills:
1. Lay in the shadows with a large fuzzy brush. In photoshop or the Gimp I'd suggest doing this on a layer with the blend mode set to overlay.
2. Lay in the highlights with a slightly smaller fuzzy brush. Avoid sharp edges. You want hills to be rolling, and in contrast to the sharp peaks of a mountain range. Again, here I've done this on an overlay layer.
3. Add colour (here I'm a layer with the blend mode set to colour) and leave the hills slightly browner than the flat plains. That helps to differentiate them - and means that even with subtle light and shade they'll be easy to read at a glance.
A couple of other things to keep in mind:
• Lay in the rivers first. As rivers drain the water out of hills, they will determine where the hills should go.
• Less is more when it comes to shadows and highlights here. Your mountains should have the darkest shadows. Make sure that your hill shadows are quite a bit more subtle.