"I wouldn't describe them as tricks. You do develop very quick reflexes of rejection and editing of your own stuff. When you're young, you're so fond of what you have created, because it takes a lot of effort to extrude this onto the page or onto the screen. You're very fond of it, even if it's wounded and you're barely alive, you still have affection for it.
But as you get older, you learn how to throw it out without much thought, without much pity. You look at a piece that you've written and you take those first three paragraphs, and you dump them. You just rip them out. Usually that's the part that needs to be thrown out, the big windup, the big introduction. The first page almost always can go. You learn to do that without regret."*
The sooner you get to that point the better. Some people never get to it. An acquaintance who shall remain nameless still can't help giggling whenever he reads something he's written and he can't bear to cut a word. Unfortunately, but perhaps not coincidentaly, he hasn't gotten very far as a writer.
What can you do to be better at this?
1. Put the material away for a week or two.
2. When you critique it, do so in a different location from where you created it.
3. Read it through a few times. Usually that cures you of being entertained by it any more.
4. Start with the big stuff (structure) and then move down to the details.
* from an interview on TomPeters.com: http://www.tompeters.com/cool_friends/011353.php