Great topic! I am taking music theory, which involves ocassionaly composition projects - mostly simple, but I have a tough time with them! By the time I put something together, work it all out, practice it so I can play it, do the analysis, etc, I have heard it so many times that I am sure it's been done before! It's an awful feeling, especially just before playing it and turning it in!
I am lousy at thinking of melodies, but I am good at inventing rhythmic themes, and complementary lines, which saves me! If you have any tips on how to think of nice melodies, I'd sure like to know!
As much trouble as I have with it, it's a bunch of fun to build a little piece! (mine are all dinky)