Here's a few progressions I'm coming up with at the piano here...
C -> A -> Ab -> Db
C -> Fm - > F#m -> Ab -> Db
More of a pop modulation... C -> C/Dm -> Ab/Gb -> Ab -> Db
C/Dm means a D minor chord with the C root underneath it.
For a quick change in mood C -> C(#11) -> G -> Bb -> Bb(#11) -> F -> GbM7 -> Ab -> Db
C -> A -> Ab(aug) -> Ab -> Db ... or just C -> Ab(aug) -> Ab -> Db
I hadn't looked at the piece, I want you to do what YOU want, it is your composition. So, I made a few suggestions here, if you would like to take any of them and play around with them and see if any fit, go ahead. Don't be afraid to repeat some parts, for instance doing a C -> A twice to see if it works better. Just play around with them, if they don't help, they don't. If they do, well jolly good.
You could also try modulating into a different key first and then trying to get back to Db from there. This is called tonicisation if I am not mistaken. I can't spell it, though...
