I've actually had a tough time with this. All my bikes have drop bars--and I hate having the bell on the stem. For commuter bikes I've been attaching the bell to the right drop, a bit below the brake lever. If I have thick bar tape, I wrap over the bell's clamp. With thin tape, I attach the bell over the tape. When I am on the hoods (95% of the time, probably) this allows me to easily flick the bell with my thumb. I pretty much never go into the drops on these bikes, so it's not in the way. It looks like this: