Actually, Sean, most masters had the form pre-planned for the most part. I believe it is best to have a plan for the piece before the actual process of composition is begun. This is the way Beethoven, Mozart, and many, many others do it. Make rough sketches of what form, motive, orchestrations, etc. you are thinking of doing.
Now... onto the fugue! What a form to start off with, Styxx. A proper fugue is probably the most difficult to compose using the rules of 18th century counterpoint. It is much easier to do now, but harder to make it sound good. A true fugue has an interesting difference from a canon or straight imitation. When the subject is played by the second voice it is a fifth up and every interval must be the same as the original subject. So, composing the subject has to be well thought out or the piece can get very strange. Bach considered his fugue writing to be perfection and proclaimed that, as a standalone form, no more need be written after his. And, as far as I know, no one big has. There are plenty of fugal sections in other forms (see the fugal sections in Beethoven's Symphonies), but none by themselves.
I will say, that using modern harmony, the fugue could be born again, but I still use it mainly as pieces of movements. I studied Alfred Mann's collection of Fugue studies that includes the section of Gradus ad Parnassum by Fux.
Here are the forms I personally like best:
Sonata Form (To me, the funnest to compose):
Theme 1 - Main key (if in a key)
Theme 2 - Dominate key (major) Relative Major (minor) Something different (non-tonal)
repeat above (not necessarily an exact repeat)
Developement (the fun part)
Recapitulation (repeat Theme 1 then Theme 2 in the same key as Theme 1)
A B A C A B A a <-(sometimes)
Scherzo and Minuet:
A B A (triple or compound duple meter)
I stick to those forms for a lot of what I compose. One can also discuss the various binary forms used in Suites.... Allemande, Gigue, etc.