Trixa Pyswell
If the hero/heroine has a best friend of the opposite sex, they WILL fall in love.
It's just how it goes.
EDIT: But of course they wont realise that before the hero(ine) has been sleeping and 'almost in love' with some handsome person. On the other hand, if the hero(ine) has a best friend of the same sex and they're both straight, they will fall in love with the same person. The hero(ine) will always win this person's affections, however, causing jealousy and feuding, but eventually the best friend will realize the error of their ways and make a sappy speech about how you can't force someone to love you.
If one of them is gay, it will be the best friend, and they will fall in love with the hero(ine). Their feelings will be unrequited.