How do I optimize my maps for printing?

[Coriolis]

I'm preparing a small portfolio of 12 pieces to send to a publisher. This publisher wants physical prints, though they'll take image files in addition to them. I'm looking for methods and principles of getting these images to look as clear and as eye-catching as possible.

I've printed these out on a 600 dpi color printer, and I'm underwhelmed with the results. These are the problems I see when I print them out:
1. Most of them are too dark, even after I've lightened them. I'm having trouble finding the right level of brightness.
2. Contrast seems to heavy in some areas and too light in others. Many smaller labels become difficult to read for want of clarity, while the contrast on many of the icons have too much contrast, to the point where details of the parts in shadow can't be seen. Meanwhile, a lot of the lowland topographical bump mapping seems very subdued.
3. Most of the coastlines don't have nearly as sharp a distinction between ground and water as they do on the screen.

Here are some examples:

My most-recent map is a hidden mountain vale. Though the mountains that surround it are clear and bright, the land in the middle is far too dark on the printout, to the point that it's hard to see details and read labels.


A post-apocalyptic map of Michigan's upper peninsula from the Rifts RPG. The borders help to make the coast stand out a bit more, but a lot of the details are hard to make out, many of the small labels are difficult to read, and the light/shadow contrast of the city icons is far too strong. The map prints out a bit dark, and the water loses a lot of the texture and bathymetry I put into the image file.


A map of Atlantis from the Rifts RPG. The ocean prints as too dark, the swirls in the water are too difficult to see, the land doesn't stand out from the sea enough, the forest makes the land a bit too dark, and the icons suffer from the same exaggerated light/shadow contrast as in the one before.


An older map I made about a year ago, this one actually prints out decently, but the ocean is still too dark and swallows the texture work I did on it. The coast doesn't pop as much as I'd like it to Finally, the whole map seems a bit too dark.

This grayscale map I made a while back is another example. It actually comes out looking pretty good, but the labels are harder to see.

Overall, I'm pretty satisfied with how these look on the screen, and most of them have plenty of pixels to print on 8 1/2 x 11 inch paper at 300 dpi (my standard is 3375 x 2625 pixels, a bit bigger for maps with frames), but I can't quite seem to make these look as sharp or as eye-catching as they look on the screen.

I use GIMP. Are there any tricks to making these print better? Should I work with color curves? The brightness/contrast tool? Or should I play with the saturation? I'd really appreciate some advice and pointers.