Also, be nice to Marelene. Giving the flower to Marlene instead of Tifa scores big points for Barret. Another thing... in split up situations, talk to the person you're trying to woo first and, if you can, the others not at all.

As for Yuffie, it's of course the same. Yuffie and Barret are harder to get, because there are markedly less opportunities to be nice than there are with Tifa and Aerith.