If you have a strap on post level (levels in 2 directions while clamping onto the post with a rubber band for about $5) and a pneumatic nailer, you can level the post yourself and shoot it with the gun, which beats banging on the post with a hammer trying to get a nail to go into it.
Tell me you are using actual pressure treated posts, not just the "stained" treated ones that don't have any penetration marks on them.
To make your posts last longer, place a little concrete in the bottom of your hole before you put in the post, then set the post, then pour your concrete. By the time the concrete is in the hole, the post will have moved again, so you'll have to fine tune it.
Lastly, use builder's string to run a line along the top and a foot away from grade for the entire run of the fence line. You want the two strings level and right to one side of all of the fence posts. This will make your fenceline nice and straight.