Years ago I realized that, as a US citizen, I was only getting entry stamps on a somewhat random basis, and I also realized I was running out of my extra pages. So now I just ask politely when I hand over my passport:

"Could I ask a small favor? I'd like to request no entry stamp, to help me conserve pages."

I started that 3-4 years ago and I haven't had a single entry stamp since. If you are friendly and polite about it, it also has the side benefit of establishing that you are a nice person, and not one of the grumpy jerks behind you.

I imagine the same works in reverse if you want some extra stamps...

-Jim