Use control and F to do a search on this site. Try fence and maybe wood fence. I and others have posted on fences.
I'm not sure cement around the side will help much with sinking but I'd bet if it has been in there for a while then it sank as much as it's going to. You need to post back with more details on what is going on with your posts, how old, and what you are trying to do.
If you are fixing a bad gate, then my post you'll have to search for explained about using taller posts with a brace across the top.
Without knowing more, I'd say replace it, put gravel in the bottom of the hole, and tamp it good and look at my other post. The May Family Handyman magazine explained one way to jack a post out of the ground.