If you're a man proposing to a woman...
... and you're older:
Place the most expensive ring you can find in a box made of pure gold. Keep the ring safe from damage by packing it with hundred dollar bills. Fly her to Paris on your private jet and propose at the top of the Eiffel Tower - but only after paying everyone else to leave so that you have some privacy.
If she still says no, remind her of the weekly allowance that you so generously provide her.
... and you're younger:
Don't bother. She just wants you for your body and proposing will only accelerate the process of her dumping you for a younger, better looking man.
Instead, focus on milking her for every penny you can get out of her before she does that anyway.
... and you're a romantic:
Firstly, be honest with yourself. If you need my help, you're not a romantic; you should have already come up with some sappy garbage on your own.
With that out of the way, buy a fake ring. Don't worry, the money you save on the ring will go towards the rest of the proposal.
Surprise her in the morning with a breakfast in bed, then take her on a romantic drive through the country. Skip this step if she has a fear of cows or trees.
Treat her to dinner at her favorite restaurant, at the table with the best view. This will cost you, but you want to do this right. Don't you? Of course you do. So pay up.
Now take her to the spot where you first met - be it in Italy or Antarctica - and propose by the light of the first star. If she says no, kill her, dump the body and find someone who really deserves you.
... and you're only doing it because she's dropped hints for the last five years:
Use a ring you found in a cereal box. It won't matter. She's so desperate to have your babies you might not even need a ring. If you're willing to run this risk, go for it.
Alternatively, run away screaming like a little girl and hope that she doesn't track you down and try to propose to you instead.
If you're a woman proposing to a man:
Don't bother. Trust me, if your relationship has reached a point where you must propose to him, he's just going to say no anyway. Or confess to something that you're better off not knowing.
Either way, dump him and go make a profile on Lavalife - it worked for me!
~ problem solved ~