Well, how about some general guidelines that will immediately make maps look a lot better:
1. The cardinal rule: .jpegs are the enemy. Unless you want to spend hours filling in every slightly off white pixel on the map, don't use a .jpeg for a basemap. You'll end up with the characteristic 'fuzziness' of the borders and lots of white bits showing through, which always looks bad. There are lots of .png basemaps on the wiki which you can use instead. Saving your file as a .jpeg is also highly unadvisable until you get to the stage that you've got something with a textured background and all that sort of stuff (when it's essentially an picture).
2. Consistency/functionality: Make sure you've not missed any islands, that your borders are all the same width throughout as the map you've edited (usually single pixel width for our basemaps), that you're colours don't clash and are distinct from eachother and that any labels are in a legible font.
3.Editing a blank .png basemap is good. editing a historic date basemap is better. Try and find one on the wiki that is roughly the date which you're aiming for (WWI, WWII, 1760s etc.), and work from there. It's far easier to produce something which looks good if you're starting off from the correct borders.
4. However, just filling in whole countries or part of countries will also look a bit odd. Try following rivers, mountains, major administrative subdivisions or linguistic borders.
Beyond that (and this should be considered more than sufficient for posting in the main thread IMO.), the best tip is to start with a scenario and develop the map from that. Secondly, research: find out why borders are where they are, and what the aims of each side in a war were. But this is where it really becomes open ended.