In order to develop consistency, power and spin on your serve you must use the Continental grip. 

For the majority of tennis players, the Continental grip is very hard to get used to. It’s really not that difficult, simply different than the grip most people hold. The problem starts when players initially start serving with the forehand grip. Then the longer we serve with the wrong grip the harder it is to change. Eventually attempting to hit spin, speed and variety becomes a big challenge.

Get your hand accustomed to the uncomfortable serve grip with a few simple drills shown in the below video. The more you use this grip (for example, for volley, overhead, under spin shots and exercises) the easier it will be for your serve!

Also, read more tips on the serving grip in Lisa's article in the NY Tennis Magazine. 

You can also use ServeMaster to get you accustomed to the feel and hand placement of the Continental grip. The ServeMaster has two unique “V” shape markers for easy hand placement for right handed and left handed players. Place the V marker between the thumb and forefinger on the grip marker for a general Continental serve grip.