This is a question I've had for a long time but I've never really found the answer other than following some suggestions on chord sequences like 1,3,2,5 and 1,4,5, but I'm sure there must be another way of coming up with a sequence of chords that work together.
I usually write my melodies first and try to work out what chords work with it based on the notes I'm playing. Occasionally I try to write some harmonies first when I compose, but with either approach, I'll reach a point where I'm dumbfounded on where I can go next.
Let's take a simple example, I'm writing a comedy orchestral track now starts with C major, then goes to Gb major (essentially shifting key). I've written some lines on top of those chords, but now I can't find where to go next.
Is there any musical theory I should be drawing on to find out where to go next?