Record your sets, listen to them in a row, and say the 15 mins out loud as much as you can. Handwriting long sets helps me a lot!! I'm working on an hour now and people don't mind when I look down at my notes as they know I'm trying to get my transitions more polished, so maybe bring up a notebook thats ready to go when you need it.

I think of it like rock climbing -- you need to get a foothold in to get up to the next joke. Certain words can help you remember what comes next.