Rules of Engagement
I'm talking about relationships here - engaged to be married. I have recently been through hell over this issue and it can't be overstated that the most important thing when popping the question is to be absolutely sure that it's what you want to do. This might seem obvious but trust me, it's something that should be remembered no matter what.
This means Don't do it because he or she wants too. Don't do it to shut them up and don't do it to make them happy. Do it to make you both happy, or you'll wish you hadn't said anything at all.
Oh, and it's not a good idea to do at a time when you can't afford the ring either... /me cringes/i>