Matching tempo and key
A loop is recorded at one tempo, so set your project to its BPM or time-stretch it, and check that it ends exactly on a bar line. If it ends halfway, the real tempo is half or double the label. Pitched loops also have a key: play them over your bassline, and stay within two or three semitones when you transpose them.
Making a loop your own
Slice a loop at its transients and replay the slices in a new order, filter or re-pitch it, or layer it under your own drums. The more you transform it, the less it sounds like a stock sample, which also makes it less likely to collide with someone else's track in fingerprinting systems.
Licences on loop sites
Free loop communities often let uploaders choose a licence without checking that they own the material, and many loops are for non-commercial use only. Sound Server reads the licence for each loop and never rates an unverified uploader's claim better than check the terms. Use the Commercial-safe switch if the track may be released.