Things to work on to develop sidearm velocity:
1. A strong finger grip against the wall of the rim with your finger pads. Usually this is the index and middle finger pads. This can be done several ways and there are sites with pictures to help you here. The important thing is to use the pads directly into the wall and not the side of the fingers.
2. Use as quick of a whipping action as you can. Keep your arm loose for as long as you can.
3. The whipping action should cause the weight of the disc to pivot around where you are gripping, and rip itself from your grip. This is substantially aided by your wrist motion. When your wrist stops, the disc rips out. A really quick wrist motion to a steely stop will produce the best results. This must be in concert with a good strong grip controlled by your thumb.
4. Power comes from your shoulder turn, but the main acceleration, power transfer, and resultant velocity come from the whip which is your arm, wrist, disc pivot and grip.
5. Until you develop an efficient whip, there is no point to developing more power. It will just make you sore and tired. Any loss of quickness in the whip or slip in the grip will fail to transfer power into acceleration and escape velocity.
6. You don't have to pull through your whip with your shoulder at maximum speed, but you do need to pull with maximum power as the disc is being ejected. This should cause you to take a pivot step to follow through just like a properly thrown backhand.
7. A loose bent elbow is easier to produce a whip with than a non bent tight elbow. However, just like pitching, there are different styles that work for different people. The rubber arm technique is most efficient, least tiring, and least physical.