I improved my speed by working on technique...it is pretty amazing the speed an inflexible middle aged guy can generate...wrists, forearms, fingers and body core are all good areas to focus on...I also like squats (no weight, just squat and stand up)...seems to help in maintaining my posture late in round...really though, technique is king...I swing much "easier" now than I used to, use less energy and feel fresh at the end of a round as opposed to how I used to feel...and I hit the ball farther now
Cheap answer get a weight do nut about a couple of dollars. Use an old club with the do nut and begin swinging easy and work your way up. Then repeat swinging on your opposite side to even things up.
If you have about 100 dollars get Vijay's 'Swing Stik'.
I normally strength train and do some treadmill running. Swinging exercise is as much for making the groove part of muscle memory and also to work on timing. The object is not to get stronger like pumping iron, but to increase you swing speed, timing and to make the swing path a natural feeling. If you are too vigorous with a weighted club it is possible to injure your back.
For increasing bulk strength in use a weighted pulley (bowflex) I use varied tensions and copy the swing motion slowly working all the swing muscles.
The best way to gain club head speed is by having a way to accurately measure your speed. One of the best ways to do that is to get a Swing Speed Radar. This will allow you to see exactly what your speed is, and it will show you what works to increase it. I got my SSR and tested my speed. It was 97MPH at first, Then with some testing of all the things that are supposed to help, I got my speed up to 107MPH, My high is 118, but I"m still working on figuring out how I did it so I can do it again. If you really want to gain speed, the SSR is the best $100 you will spend.
Bump; Before you buy one, send me a PM and I'll fill you in on the details and the real difference between the SSR and SSR/TT. I have both. I measure my swing speed with the SSR/TT, and measure my ball speed with the SSR at the same time. Works great
I would purchase a heavy club like the Heavy Hitter. Its one of, in not the only, heavy club that you can hit balls with. Swing a bucket with that and then put your driver in your hand and you'll see a difference.
On a side note, make sure that you are relaxed when standing over the ball. Too many times I see a guy wanting to pound it, but he is so stiff he can't even get the club all the way to the top. If you were to crack a whip, you wouldn't do it with a stiff arm would you? So just watch for that and you should gain a few mph swing speed right off the bat!