Damp but well-wrung towel on top, dry towel to line the bin, and a hard cover for the bin itself, but not an air-tight one. The hard cover can be omitted if you're in a high volume place where the mesclun will get blown through in a couple days, but I suppose you wouldn't be asking about this in the first place if that were the case. :D
Depending on the quantity of mesclun, if it's a lot of mesclun then I'll also add a second dry towel in the middle before piling up the rest of the mesclun on top.
Paper towels will work too, though cloth are still best.
Reasoning: Damp layer on top, to prevent wilting while giving the mesclun some moisture to stay fresh. Dry layer on the bottom, to absorb any condensation and therefore prevent rot. The two in tandem IME are the most effective way to retain freshness. For uber-preservation (and if your kitchen towel budget allows for it), change out the towels every other day and downsize into fresh clean storage containers as the greens are used up, to remove the accumulated ethelenes, condensation and sulfides which encourage rot.
Also, when receiving the delivery, break open the bags and smell the mesclun. It should smell as fresh as it looks. Sometimes good looking mesclun will have a funky smell to it, not detectable unless you get your face in it, and that indicates a case further along the spoilage path, or one which wasn't refrigerated properly. -Then- you can give your purveyors an earful! :D