Despite having tried learning a good dozen or so mapmaking and drawing programs, from Gimp to AutoCAD to Hexmapper and AutoRealm, I still mostly make my maps by hand. When I'm lazy, I might search the net for few ready-made ones, or buy pre-made adventures.
In the past, I used a style similar to old D&D hexmaps, or the generic "fantasy map look" that is well-presented in this thread. Lately, I've started using real world geographical and orienteering maps as a baseline. I find this modern style more informative and easier to draw than elaborate symbols. I also use ASCII-style numerical and alphabetic notations on grid paper.
If I ever get a CAD program with good key shortcuts, I will start drawing my dungeons according to real world technical standards. Because I can.
My hints for mapmaking:
- If you ever find a program with good key commands, forget the mouse ever existed. Instead of drawing, learn to use XY coordinates and make those objects go exactly where you want them to!
- Forget looking good. A map is for telling where stuff is, being a work of art comes strictly second. Emphasize only key items that map is for such as nation borders, roads and rivers, and leave all needless detail OUT!
- A table or two for random item generation accompanied by a few rules of thumb for how they affect their surroundings (f.ex., rivers flow downhill, mountains gather moisture and stop clouds etc.) will help you seed your world with intereting stuff.