Peter McNeill writes: 'All I can say is just keep practicing.'
Pie Girl, as simplistic as it seems, this is the best piece of advice anyone can give you. Just keep experimenting and at some point (unless you have a genuine impediment) it will kick in. At one time, I couldn't whistle. As a child I just kept trying and now I can whistle really well. Same thing with rolling Rs. Now I can roll them like an Italian, in the throat like a Parisian and I can do the weird 'r' sound in Czech and Polish where you crush the roll at the top of your mouth. All these things were learned, so I urge you to keep on trying when you're just doing anything else and it comes into your mind, without your instrument. I remember learning to roll Rs actually, and I started by just going for one flick or bounce of the tongue and then built it up to more and more.
There are some videos on Youtube that might help. Trumpet Sound Effects (7) : Flutter Tongue - YouTube. But the bottom line is the same as the top one - just keep practising.