I buy them bagged with most of the air squeezed out (good quality with all the dead stuff and skins removed). Kept fridged like this, they will be fine for 3 to 4 days.
If I buy more than I use for the session, I rinse them and find an airtight tub just big enough to hold them (the ones with 4 clips, one on each side are best). Once in the tub, I fold a sheet of two of damp newspaper and put it on top and fit the lid nice and tight, so they are well packed in. I then turn the tub upside down and put it in the bottom of the fridge.
Storing them this way, I've kept them and used them (and caught the same as normal) for 8/9 days from purchase.