Creating Fills

I am working in Improvising Study 1...
Creating fills is a major source of difficulty for me, at least one's that sound half-way decent!
Any special tricks? Or is this something that just comes with time and practice.

In another FORUM TOPIC about improvising I saw a comment from Winslow that stated trying working with 2 note combinations...trying to come up with as many different patterns as you can. Would that be useful to try when creating fills as well?

Thank you.