Aww c'mon I know this is not technically allowed and is something that is frowned upon by everyone, however you can do it without it looking like a bribe. Below are some of the things I did to get the refs on my side. Yes this can be called cheating and manipulative however I am a very loud coach which can grab negative attention from the referees. 

Compliment The Ref's Shoes: I have done this a few times with the refs in Toronto, Some of the referees in Toronto have really cool shoes. So instead of keeping my thoughts in my head I vocalized my opinion on his shoes. I have no idea if this worked but hey anything and everything can work. 

Buy a Coffee: There are a few referees that I have become really good friends with, there are 4-5 referees that if I know I am going to have them I will buy them a Coffee or Iced Coffee (pending on weather). This is one way to get on the good side of the ref, like I always say the best way to get to someones good side is through his/her stomach. 

Be Professional: Being professional is the best way to bribe a referee and you don't even have to spend any money. If you hate the referee because he calls a tight game don't totally ignore the referee at the beginning of the game, go up to him say hi and shake his hand, even ask him how he's doing! Also if a referee is making your players mad during the game make your players NOT TALK TO THE REF INAPPROPRIATELY! Also when talking to the ref you CANT act like a child and complain about the call he just made. You need to talk like a grown up and ask the ref if he could explain his call! 

I hope the above tips help you with your next trouble referee.