It can get annoying tracking all the little errors. At least if the game starts up correctly and the campaign is missing I know that the problem is just a syntax error in the startpos file.
I try to always work on one section at a time and then start up the game to make sure that the campaign is still working. Tracking down a single letter syntax error can be quite a handful if you basically have multiple sections of the startpos file to sort through.

My most common mistake is using a faction name instead of a province name and vice versa (i.e. FN_NORWAY instead of FN_NORWEGIAN).