Do not have stiff arms
While you practice striking the ball, you should make sure that your arms are completely free of tension. Allow your arms to hang freely without holding a gold club and touch your forearms to feel if they are relaxed or not. Most PGA experts believe that a golfer cannot improve his/her swing, until the arms are free of tension. If a player looks to keep his/her arms stiff and straight, he/she will not be able to put proper force behind the shots.
One of the most useful practices for improving speed is swinging with the club held upside down, pointing the handle towards the ground. Hold the club far down on the shaft to ensure that the handle does not touch the ground. Now start to swing the club faster till you hear a swoosh sound. Try to make the swoosh more audible with regular practice.
Along with practicing speed, try to make your swing as smooth as possible. Make sure you don't jerk with the swing. The smoother the arc, the more force is transferred into your shot.
Set little benchmarks for yourself and try to improve each day you practice. Most of the amateurs swing their golf club at approximately 85 MPH; whereas, the professionals swing at about 110 MPH or more.