Transitions, and variating on your main melody

I seem to have the most trouble with transitioning into a slower part and/or a faster section of pieces I compose.

I also seem to have trouble with varating on a main melody I decide to go with.

Would it be possible for me to get feedback on how exactly you would lead a piece into a slower part and/or a faster more upbeat part?

As far as varying a melody goes, it's definitely worth having a look at how other composers have dealt with the same issue. Two examples that spring to mind are by Beethoven:

1. Diabelli Variations (piano)

Generally, tempi stay more or less the same throughout a section of music; that's why larger works comprise movements of varying speed. As far as varying themes go, I suggest you have a look (as jada suggests) at lots of different self-titled 'variations'.

Bach's Goldberg variations, Beethoven's Diabelli variations and Rachmaninoff's Variations on a Theme by Paganini showcase countless ways to alter a melody.

Shouldn't this be in the Advice and Techniques - Young Composers Music Forum forum? :P