Hey Superfly,
Yes I do have a tip or two.
Don't count on trying to space the posts perfectly on equally spaced centers. You prolly can't get them perfect, but you can get them within a couple inches. (You will never notice a couple inches if you stand way back and look at it.) But dont be sloppy about it, try and keep the posts as closely centered if possible. When you set the posts you can use a decent size hammer and block of wood to tap the bottom of the post a bit to move it around some if you need to. And you will need to. Quite often.
This is what I did. (a tip from a youtube I found). Set the post spacings 4-6 inches shorter than the length of the horizontal rail. When you go to install the rail, set it up to be post and mark the rail at the centerline of the post, and then trim off the few inches on the end of the rail. This method will allow each horizontal to center up on the post. Then butt up the next horizontal rail to the one just installed. Mark it on center of the next post and trim off the end... and so on. When you are finished, every horizontal will be in the center of each post and it will look really nice.
If you dont do this, and for example a span is 2 inches too wide, well then its just 2 inches too wide and you have a problem. Its hard to stretch a board, pipe or whatever else type material you are working with. You will have to go buy a longer one, and cut it off to length to fix the problem.
Another tip. Dont skimp on the post depth deep either. The depth of the post is critical to the stability of the fence. Especailly if you get some fairly strong winds on occasion.
a 40 mph wind could easily blow down a solid wood, metal, PVC or any type of solid fence with posts that are planted too shallow. Can't tell you how many fences I have seen blown down where i live because the post was too shallow. The post either comes completely out of the ground or they snap off just above the ground. You can rent gas powered post hole digger, they are worth every penny, and more. Trust me on that one.
Also a post planted too shallow will not stay plumb. It wont be long and they will start to lean over due to the weight of the pickets, pulling on them, and you end up with an ugly leaning fence that will probably be blown down by the wind.
I just noticed you are from Wisconsin. The post needs to go deeper than the frost line, or its subject to heave out of the ground when the ground freezes. Check what your frost line is and use the recommendations of how deep to plant the post based upon that. (I suppose you prolly already know that though).
Hope this helps,
any tips for keeping the spacing right while the post setup in the concrete?