And don't allow yourself to do them in advance, because once you've built up a margin of error, you can quickly find that you don't get back into the habit once you've used it up. Making up works better -- it puts in an inherent punishment.
Revision can be fun. I've never found a really good measure -- words changed (plus or minus) is not much help if some passages don't need much change, and pages done, if some need a lot.
The worst effect for me is when it results in my only writing once a day, even though I could do more. I look at the desk and think that I've already done it, and the initial reluctance that inspires quotas is back full force, and not to be budged.