Question about raising/lowering saddle
 

I'm on a new Trek 3900 and I'm quite new to cycling. The bike is about 10 days old, 73 miles. I've tried adjusting the saddle position up and down, but when I complete a 5-10 mile ride, the saddle is all the way down again. The quick release piece is VERY tight, almost impossible to loosen by hand, although I do. And it is VERY hard to tighten by hand once I find the position I want, but somehow I get it closed again. Then I ride, and when I'm finished i notice the seat is once again all the way down.

Suggestions?

Is there any grease on the seatpost? It might be worth a try to wipe the seatpost clean, just to be sure. Also, is the quick release positioned correctly at the top of the seat tube? It seems very strange that the quick release is that tight and yet the seat post is losing height...

Since the bike is so new, you could have your bike shop take a look for you if there is nothing obvious that you can see.

The post should be greased.The post might be the wrong size.

why grease the seat post?- I have noticed that new bikes come with grease on the seat post but it seems counter intuitive particularly where the post is slipping

I was just thinking the same thing. Quick release seat post clamps are often quite painful. The post on my Klein used to slip when I first got it. More grease on the seatpost solved the issue, though I keep meaning to buy a non-quick release clamp anyway.

Without grease, the post will freeze inside the frame (due to the dissimilar metals).....then you'll never be able to adjust it.

That could be good.

Since it's a new bike, have the bike shop check the inside diameter of the seat tube with a micrometer, as well as the seatpost diameter. It's always possible it got assembled with the wrong post.