Seatpost won't stay in place inside the seat tube
 

The seatpost was doing 'fine' earlier. Since yesterday after I cleaned up the seatpost, the inside of the seat tube, of course re-applied new grease on the post then the post will never sit tight in the seat tube anymore.

I did tighten the seat bolt. I was thinking maybe the grease is causing the issue but the post will get scartched up pretty bad without grease on it. What's going on?

I had the same problem. The threads of the binder bolt wasn't greased and was damaged from over tightening. I had to get a new binder bolt and greased the threads before tightening.

Carbon post? If so wipe off the grease and try it. It it still slips, try carbon paste.

Aluminum post? something is wrong with the clamp or the post doesn't properly fit the frame. Fixes are getting a stronger clamp (I recommend the salsa lip-lock) or getting a new post with the correct diameter.

I had a seatpost on a touring frame that I *though was too small dia for the frame, but it actually just needed a stronger seat post clamp -- fixed it.

It's an aluminum post and pretty sure the size is right as I never change the post. Like you guys said, maybe the bolt? The reason why I say this because I actually broke the 'original' bolt so I stole one from my other bike and replaced with it. Does these seat binder bolts come in different length/size?

Yes, binder-bolts come in different lengths. If you get one that's too long, it bottoms out the threads before you pinch the clamp-ears tight enough.

1. take the bolt out and screw it into itself all the way

2. use calipers to measure the distance between the underside of the bolt-heads

3. measure the outside distance of the two-ears on the seat-clamp

4. is measurement #3 larger than #2???

Ideally you want a bolt that is threaded all the way up to the head so you avoid this issue.