Make sure you have the graphic positioned where you want it before you start, a little planning goes a long way. Only peel away part of the backer at a time and work from one end to the other. If it's a wide graphic, still work from one side to the other but as you go, work from center towards the edges to keep from trapping air bubbles. Use some kind of squeegee to keep the vinyl smooth as you apply it, even if it's just a balled up shirt.
If you have any questions on these I can probably expand on these.