I bought my Diamondback mountain bike about a month ago. There has been an ongoing problem with the seat post slipping. I get the seat height adjusted, and go out for a 10 mile ride. The seat will sink about an inch of the course of the ride. So my legs become less and less efficient. I tried tightening the seat post clamp. I have adjust tight enough that I have a hard time getting the clamp closed. Still it sinks. I had the seat adjusted about perfectly before the duathlon yesterday, but by the end of the 10 mile ride, it had slipped by about an inch. I went to the sporting goods store to talk to the bike guy, but he wasn’t working. He will be there tomorrow. I will stop in tomorrow night. I read on one web site a suggestion of removing the seat post, and cleaning off the lubircation. So I pulled off the seat post, and sure enough, there was grease of some kind on the post. I wiped it off with a paper towel. I also wiped out the inside the opening of the seat post tube. But then I got the idea that there might still be too much lubrication. So I sprayed a little lighter fluid (naptha) onto a paper towel, and wiped the the seat post and tube. Then I re-installed the seat post, and got it adjusted. Maybe this will keep it from slipping.