I use heavy steel threaded rod, 1" or larger, galvanized if you can get it, and I drill a hole at least 1 foot up into the bottom of the post and screw the threaded rod up into it. To do this the hole has to be the same size as the main body of the rod so that the teeth cut into the wood. I use a large vice grip to hold onto the rod as I twist it. Then I drill a hole in the concrete a little bit wider than the threaded rod. I then stick the threaded rod into the hole, into which I have poured hydrolic cement. You can buy small bags of this stuff at your home center and it is a special kind of cement that slightly expands as it sets and it sets up in a few minutes. Mix it so that it is fairly watery so that you won't have any trouble fully seating the post/rod combo. Also, always dry fit it before you mix the cement to make sure everything fits. Also, put preservative on the bottom end grain of the post, and in the hole in the post before you screw the rod into it so the whole mess won't rot etc.. You can sit there with your level for a couple minutes to make sure the post stays plumb while it sets up. Make sure the posts aren't twisted in relation to the entire run so the rails but up nice and squarely - you might want to set a string a 1/2" from the edges of your posts so that you can see that the edge of the post is parallel with the run of railings. You can rent a large hammer drill at a local rental shop that will drill the 1"+ holes in the concrete. I would run these holes at least 6 inches, and if you go through the bottom of the slab, fine, then the bottom of the hole can mushroom out and maybe go down to 1' or so. The reason that the holes in the concrete need to be larger than the rod is to give room for the cement and also so that you have some wiggle room to make sure that you can get them plumb and in line with your run.