The best proposal is no proposal!
Why propose ? Girls are indirect and they take hints quite well. Always keep them in the loop and when right time comes it happens naturally just like kiss or sex.
Now if you are asking for formal proposal when you know the answer but only want to make it more dramatic like bending on your knees in middle of movie in a cinema hall and as the hero on screen proposes his girlfriend, you just lip sink!
Oh I just gave you an idea!