I've upped the frequency of my head shaving to almost every day. I had been starting to get some bumps, but only on the back of my neck. It was really bothering me, so I ordered some Bump Patrol based on word of mouth here on the forum.
However, before the Bump Patrol arrived, I decided to try one other thing.
When I shave the back of my head, I bend my head forward (chin to chest) to get a nice close shave on the back of my neck and head. I began to wonder whether that was causing the bumps.
So this week I decided to just keep my head in its normal upright position when shaving the back of my head and the back of my neck.
Result: I'm still getting a very close shave, but no more bumps.