I'm so unfamiliar with this application/winning concept when it comes to being sweetheart. I tend to liken being a sweetheart to AI... it shouldn't be something you seek, it should be something you surprisingly get just because of who you are and what you (selflessly) do for the chapter.
Here is another sweetheart thread from not too long ago...
http://greekchat.com/gcforums/showthread.php?t=101021
And my chapter also frowns upon choosing a member's significant other as a sweetheart (because of the reason stc2005 stated.. if they break up, things could get complicated), but I know other chapters that welcome it. So it all depends.