There was a thread last year about someone not being able to drink from a water bottle while riding, and someone (I'm pretty sure it was Tulip) recommended a way to gradually practice riding with one hand, reaching for the bottle in the cage, etc. It involved reaching down and touching the bottle briefly as a way to get started. So I followed that advice for signaling with my right hand. On a safe part of the ride (i.e., flat straight trail or road with no one around me) I would practice lifting my right hand off the bar, first just a couple of inches, then a bit more, etc. until I was able to signal.

Oddly enough, after a couple of winter months off the bike, I found that this year I was more comfortable taking my right hand off the bar than I was last year. Still not as good as riding with my left hand off the bar, but I'm definitely better at it now.

In addition to signaling right turns, this has made me better at signaling road hazards on the right edge of the road while doing group rides.