The best way I've found to do this is to carefully paint washes into the recesses, darkening them. I tried GW's Carroburg Crimson, but I found that it wasn't dark enough for what I wanted. I've found two Secret Weapon Washes that work really well, depending on what you want.
Red black is a strange wash, as it is very purple when it's wet, but dries to a nice, deep red. It gives you a subtle shading on a bright red. One of the major benefits is that it doesn't tint the paint much when spread in a thin layer, which is what happens when some of the wash spills out of the recesses. This makes it very easy to repaint the raised areas and clean up the shading on the model.
Sewer Water is a reddish brown wash, and is pretty similar to the old Devlan Mud. This means that it's a great wash on just about anything. It produces a very pronounced shading, giving you deep, clean demarcations. I like the overall look a lot, but it does strongly color even with a thin coat. This means that you'll need several coats to cover any spots that the wash gets out of the recesses. This makes using Sewer Water for any light colors (such as red) very time consuming, but rewarding in the end.
Here are some examples for comparison. Both tanks were originally painted a flat Blood Red. The Rhino was shaded with Red Black, while the Baal Predator was shaded with Sewer Water.
|Red Black on the left, Sewer Water on the right.|
The differences are subtle, but it really comes down Red Black producing a softer, more red shade while Sewer Water is a deeper, more defined shade.
I like both for slightly different effects, so I'll be keeping them both around.