The decision to divert begins long before the actual event. A wise flier figures out some way to notify dispatch of his/her choice of divert points and then lets them suggest yea or nay. This begins the process so everyone has some input while also having a way out.

Example.. you begin checking wx at destination and it is going to poop. Look around at what you need to have do depart holding with to get to an alternate. Then inform dispatch of what you are thinking. They may decide on some other airfield because of re-routes, transportation or whatever but you are working the system.

One thing.. do not make a decision and then waffle. It is a recipe for trouble. IF you decide you are going to divert with x fuel, when you get down to that fuel, DIVERT. The trap is to keep waiting a few more minutes hoping the destination will open and also just after you turn to divert, your destination opens. WRONG... I have seen too many cases where you just move up to the next fix, the destination goes back down and now you are even lower on fuel and have less options.

Decision making begins on the ground and then in the air, you just have to implement them.