Use basting stitches to hold your material firmly.
Hugs n roses, Meganne

Having the hoop secure and tight, to begin with - with your stabilizer in without any pull. If you need to have something extra to make it not shift - then do so - as Meganne suggested a basting stitch will help. I got this tip from one of the websites and can't remember which - but I have be using it and it does help. Good luck - the hooping is a key to a great design. *4U

Basting stitches works like a bomb.

If you are using a large(r) hoop the sides my bend a little, just enough to let everything slip, stabiliser, material and all. Some of the larger hoops have clamps to prevent that. On KODesign.de they recommend pasting emery cloth to the outside ring. An other methos is to use a piece of the non-slip waffle stuff you use in drawers with a rectangle cut out and frame that with your material(some people use felt). Husqvarna sells them in a felt like material ready made for their hoops.

If you're not getting along with the basting stitches it sounds like you're not getting your stabilizer in the hoop tight enough. All of these suggestions should help. But I have one additional little tip. Once again get out the waffle weave shelf liner. Cut a piece bigger than your hoop by a couple inches. Lay it flat on your table and sit your bottom hoop on it. Lay your fabric, stabilizer or both over the bottom hoop and place your top hoop. Now you can adjust your placement without the bottom hoop moving. Make sure you tighten your hoop so fabric is drum tight. Unless of course it is a knit fabric..then we're talking a whole new ballgame.