One of my hugest pet peeves is when characters who have supposedly known each other for years, or they’ve had a very close person in common, that don’t seem to know know each other.  This seems to happen a lot with male characters who end up with their best friend’s sister or female characters who end up with their best friend’s brother.  It’s not the same to just make them fall in love with each other the moment they see one another out of the blue, because they’re already used to the sight.  Something needs to have changed in order for them to reassess the way they look.

What you need to make the past work in the relationship:

1. Shared memories of a few past events AND how they felt about each other at that point in time.  Did one like the other while the other didn’t notice?  Were they antagonistic toward one another?
2. When did each of them start feeling differently?  If there wasn’t a direct cause, then when did they realize the change happened?
3. The characters have to know the general modus operandi of their long time loves, e.g., bad habits new or old, tendencies toward certain actions/activities, etc.  But it’s also really refreshing when either these things are seen in a new light, or the characters discover something new about each other.
4. The characters should seem familiar with each other.  I’m not quite sure how to elaborate this, but until you establish their feelings about each other and the history of it, it would seem odd to have the characters not seem like they normally belong where they are.

Tessa Dare handled it well in Goddess of the Hunt, when the characters weren’t originally interested in each other but it was quite obvious they knew each other for years, understanding how to manipulate each other and what they wanted.  They were aware how they felt about each other then, and as the story progressed so did their feelings.

Tracy Anne Warren’s Wicked Delights of a Bridal Bed (it’s title doesn’t do it much justice, but that’s another peeve for another time) was also a good example.  The hero was friends with the heroine’s brother for years, and he’d been in love with her for that long and she had a crush on him too, but circumstances weren’t ideal.  But when the story opens, many things have changed that allow them their chance.