I find the bagged mixes wilt faster than whole head/bunch greens. One important thing about bagged greens is to check the sell-by date. You'll rarely find bags that are dated more than a week out, but try not to get one with a sell-by that's the next day or two. I've also find that they wilt faster once you've opened them. The biggest enemy of fresh greens is excess moisture, so if you get condensation on the inside of the bag, that can be a big culprit.
Buying whole greens is cheaper, and here's a great method for keeping them fresh for a maximum amount of time. This technique comes from the great food scientist Shirley Corrhier:
Break the head of lettuce (or whatever green you are working with) into separate leaves. Don't chop, just remove the core (if any) and pull the leaves apart. Wash thoroughly; the easiest way is to fill a very large bowl or tub or even a clean kitchen sink with water, swish the leaves around, and let the dirt settle. Remove the leaves and drain and rinse the bowl/tub. Then, fill the bowl with cold water and a good amount of ICE. Put the greens in the ice water and push them down to submerge as much as possible. Let them soak in the ice water for 30 minutes, stirring gently occasionally. Remove the greens and DRY THOROUGHLY using a salad spinner and/or by laying flat on a large clean towel, rolling up, and squeezing gently. Put the dry leaves in a dry ziploc bag (gallon size is good). Seal the bag, squeezing out as much air as possible.
It sounds like a lot of work but there's really only about 5 minutes of effort involved. I've had greens last for WEEKS using this method, it really works.