You need better PMs. A PM that can say no is a hugely valuable resource.

My guess is that you either don't have PMs that say no or there is a really poorly structured chain of communication (ie - you have clients contacting more than just their PM for changes) which is changing the PM/Client relationship.

It is largely about drawing a line in the sand. If you let your clients overrun you with requests, then they will keep doing it. Why wouldn't they? Get billed the same for more features? Sign me up. Luckily I have PMs watching out for me.