I'm by no way an expert but here's how I take 'em with my digital camera...
First off, be familiar with your camera, the shutter button has two stages, you can press it halfway and the camera will focus and press it all the way to take the photo. Many people complain about digital cameras having a lag between pressing the button and taking the photo but this problem is very easy to work round. The lag is the camera focusing itself so to combat it you need to focus beforehand.
To do this, just before the flight frame your photo (more on this in a bit) and then press the shutter button halfway so the camera focuses on the rocket, now, if you keep the button half-pressed the focus should stay locked untill you release. Remember to focus the camera on the rocket/pad and don't try to focus on the sky coz that ain't gonna work! Experiment and see what does work.
The moment you see smoke coming out of the motor press the shutter all the way down (it should still have been half-pressed) and the time it takes for you to react will mean the rocket has had time to spring into life. If you want the rocket higher off the pad then try and slow your reaction slightly or put a short pause in.
Framing your photo is important too, think beforehand what shot you want, perhaps just clearing the rod etc. etc. Remember when framing the photo the rocket will be a few feet higher than what it is on the pad.
Again just experiment, you're using digital so you can see straight away if it's working, don't stick exactly to what people say just develop your own technqiue with whatever works for you.
HTH