Here is how I have always done it.
1. Drop it down into the small ring (or where-ever the deralleur defaults to, on some mountain bikes, it is the largest chainring)
2. Adjust the barrell adjuster (if you have one) so that it gives the cable as much slack as possible.
3. Undo the cable clamping bolt on the deralleur, and use an assistant with a pair of needlenose pliers to gently remove all the slack from the cable by pulling on it.
4. Re-torque the cable clamping bolt.
5. Run through the shifting, and add tension to the cable as necessary.
Worked for me...