stretching isn't really about gaining muscle mass, but more to prevent injury to muscle and joints and loosening up the muscles you'll be using. Like 3onDubs mentioned above, I'll start actually by doing a set of exercise/s with really light weights just to get the muscles started and warmed up, then do dynamic stretches to loosen up the muscles. This combination warms up and loosens my muscle and gets the blood flowing there to prep for the workout.
my old gym teacher showed us pics of one of his friends after a particular accident: the guy started doing bicep curls without warming up. although the weight probably was a bit out of his zone, still shows the dangers of not properly stretching as his bicep snapped and rolled back up to his shoulder. post surgery and healing, the pic showed the injured arm with almost total lack of muscle. obviously this isn't common, but is a risk nevertheless.
after workouts do static stretches to make sure the blood flows and isn't all concentrated in the area worked, and also to make sure the muscles don't stiffen too much around the area to decrease your flexibility.
Former King of Stock...
E90 325i, Jet Black: Done with
Current: E92 328i, Sports Package, 335i wheels, Power Seats, BMW Assist/Bluetooth, IPod/USB, heated seats, //M Short Shifter
Future Mods: 19" CSL's, KW V2, 15mm spacer, tints