first of all, fuck jelly. jam is where its at, with the fruit and shit in it. second of all, there are 2 schools of thought on this. if you are using crappy bread that falls apart or you are going to be storing the sandwich for later consumption, you apply peanut butter first, but more thinly and on both slices of bread. that way, the jelly doesn't become absorbed into the bread when you go to eat it making it all sticky and shit. on the other hand, if you are eating it right away and it is some pretty robust bread, you apply the jam first, because it is easier to lick off the knife completely. then you do the peanut butter.
also, they must always be cut diagonally. if i ordered a pb&j in a restaurant and it came sliced not diagonally, i would send it back.