How do you buy those greens? If in plastic bags I can recommend to keep them closed for as long as possible. Most of the time they are packed with a high amount of nitrogen and have microscopic holes in them that allow just enough humidity/oxygen transfer to keep the greens fresh for the longest period possible.
William James wrote:
The company that grows them en-mass in holland grows them without soil, so there is less of a chance they ever encounter harmful bacteria. Another factor is the freshness you had from the beginning. If you're buying them they could already be X hours old, and stored in X conditions. If you're cutting your own, you know how fresh they are. But I don't think bacteria is really the issue.
How do you buy those greens?