a 20-30mm ballast is best... not just sand/cement mix.. and as for the ratio a good 4 ballast to 1 cement is fairly good...  if post is about a 4" square job, then look at using 2 bags of (bulider centre-not diy shed sized) ballast per 3 posts, you should allow about 4-6" around each post, if you are going to put posts about 12" down that is..
soak the ends of the posts for about an hour or two  in a bucket of preservative, and a real good tip is thus.... do NOT put the post onto concrete.... put the post onto a bed of 20-30mm stones as this will allow full drainage of any water that gets between the post and the concrete infill, and not form a bath for the post which speeds up the rotting about 100%

hope this lot helps 8)

good tip that about the stones Wolf most timely as I'm about to set in two dirty great 6" gate posts this affternoon for my aunty, what a nightmare digging the old ones out last weekend, whoever fitted them last time meant them to stay put, they were buryed about 3 foot in the ground in a hole that must of contained the best part of half a yard of concrete they had even drilled through the posts and put a load of rebar through the hole too, took me two days to get them out and now as I have left what look like two bomb craters I have no option other than to do the same when replacing the posts.