take it outside as often as possible... that's really about all that helps.. give him treats when he goes outside... and scold him when he does inside..

make sure to watch him and if he goes to the door immediately get up...

u don't make a dog wait when he's doing what u want.. i've waited.. they won't sit at the door long.. lol...

make sure to pet it and tell it it's done good while it waits for u at the door... treats aren't always the best idea, cause then they expect it every time... praising it for doing good works too so... mix it up..