Man, I really think that this is a lost artform in today's major tournaments. But, I love doing it. It's a huge adrenalin rush. Walk the field and look for lanes. Line up trees, bunkers, bushes, whatever between you and your opponents. Don't be afraid to take your time. I've heard that the human eye picks up fast movement better than slow movement, and I believe it. Be patient and don't shoot too soon. Make sure that you are dictating what you want to do, unless of course you are caught.
My brother pulled off a sweet crawl in an APL tournament last year at Urban Assult outside of Pittsburgh. If any of you have ever seen the Star Field, you know where we were. He got on top of one of the spokes and just took his time. Some of the bushes were super thick. He was able to get about 5 feet from the guy before he let him have it. He admitted later that he really didn't want to get that close, but with the density of the brush, had no choice if he wanted to get his paint through.
I had a good one last year. Playing the 10-man division at the Zap Amateur open in the semi's against Tippmann Effect. I was on the 50 on the right tape (a wooded field) with a team mate about 40 feet to my left. I crawled half way out, told my team mate to look at me (he recognized what I was doing just by seeing my body laying on the ground and half of it forward of my bunker) I slowly made my way up until I was about 10 feet from their back line. There were two of them, and I was hiding behind a board that was nailed to a tree. The board was about a foot high (the top piece to the bunker had been knocked off of the bunker) and six feet long. Man, I have no cover. I lined up on the first guy, and damn it if I didn't shoot the only little bush between me and him. Well, I gave my position away and lived in there for about five minutes before time started running out and I got up to bunker them. I'll tell you, I had no business getting to that bunker or living in it, but it all worked because I used my surroundings. You just have to do it to get good at it.