Through a lot of trail and error, I discovered that I put a manuscript to the side for at least two weeks after I finish--a month is even better. This is the time when I go work on something else, absorb myself in some other project. Then I can come back to the original story with a fresh set of eyes--as well as a new prospective.
Most of the time, it lets me pick up flaws that I wouldn't have otherwise seen with eyes that looked at the words for however long it took me to write it in the first place. I have to separate myself from the words in order to see them clearly.
For the most part, it works. There are always things a beta reader or a critique partner is going to pick up--you just can't catch everything. Mostly because no matter how many times you look at something, you're always going to overlook something, or see it the way you want to see if instead of how it should be.
Writers, what about you? Do you have an tried and true techniques for gaining a little distance from a project in order to make it better?