Well, a way to get nice controlled lines with a mouse is to use paths (You'll often hear Adobe software users talking about the "pen tool" which is the Adobe name for it)
In GIMP, choose the Paths tool (the fountain pen and funny looking lines) it takes some getting used to so play with it a bit. When you click you create a control point through which the path will run, if you drag, you will create a second control point which will pull the path toward it in a curve. Keep adding control points to make the path. You can drag the existing control points around to refine things.
Once you've got a path you like, select "Stroke" to draw the stroke into the image.
If you do a lot of work with paths, you may find a vector graphics editor like Inkscape a better choice. This has a somewhat different interface for paths, and eliminates the "stroke" step so you can just edit the visible path directly, but otherwise works the same way. It's also a great tool for adding text to a map. I use it for just about everything, but it's possible to use it in conjunction with GIMP if you prefer.
Both GIMP and Inkscape have tablet support so that might be something you should consider saving up for.