Note: This is stuff being written in 2014, but stuff which I always followed right from the beginning of my back-friendship with school friends. Although the title says girls--because these thoughts struck me only when I became part of a group that also had girls--it applies to every friend.
never talk about another girl or guy to them
unless you are discussing something bad :D
don't tell everything you feel, wait for the right time esp. the good things. Something like using weapons in times of crisis.
when you feel like buying them something, don't sit with that thought; go ahead and implement that.
don't plan anything, it will fail: listen to your heart and mind.
don't hide misunderstandings for long, you will end up hurting her.
how she feels is how you feel she feels, if you listen closely enough