You should only say yes if you truly love the person and if you truly feel that you are mature anough to handle a marriage.
I've turned down a proposal before. Basically, we were both very young, and it more of a "reuntied" situation. We met up and before I knew it, I was in a candlelit dinner with red wine & steaks at his relatives house, placed on the spot, and being referred to as his "fiance". It was a beautiful evening, but we were long distance, both in school (no careers)and I was not looking for a marriage at that point in my life. It felt awful having to say it
, but anyway, after I said no, we went our separate ways.