This is basically simple but prep time can be time consuming. Before I start I must say that I'm not gay, but this technique is a great turn on.
First get a rolled up duvet and fold it in half thus creating a pair of legs. Then slip the 'Legs' into a nice pair of jogging bottoms (nylon is best as it doesn't soak up as much)
Secondly get a couple of extra large balloons and inflate them to the size required (not too big as they are to simulate buttocks) when inflated, slip them into the joggers in the bum region.
Last and best of all hump the living daylights out of your balloons, this gives the feeling that your humping a copy of yourself and is a real turn on.
An alternate to this is to cut a hole in the joggers about where a ringpiece is and insert your cock into the hole (with plenty of lube of course) it does make a mess but it's great fun!!