I've found that keeping around a daily list of tasks I've finished (and chucking it at the end of the day) can provide a sense of completion and accomplishment. The effort required to track this (usually just a list in Notepad or Excel) does require some overhead, though, and I wonder if it's really just a false sense of reward. After all, number of tasks accomplished doesn't prove that they were the most important or right tasks to be working on.
I'm curious, how do others who are finding success with GTD handle their completed tasks? Just deleting them right out of the context lists? Or keeping them around for awhile as a lingering record of activity completed?