I grew up making peanut butter and jam sandwiches (PB&Js) using the simplest method possible. Spread peanut butter on one slice of bread, spread the jam on the other side and combine. The problem with that is the jam can make the bread soggy and can leak out after you take a bite. Reddit user ChickenMcFail uses a trick I call the "Jam Pocket Method".
Basically, what you're doing is spreading a light layer of peanut butter over both sides and then creating a peanut butter "ridge" on one slice. Add the jam to into the pocket formed by the ridge and add the other slice on top. The peanut butter keeps the sandwich from getting soggy and as long as you don't squeeze the middle of the sandwich you shouldn't have any leaks.