Personally, I use plastic cling wrap and a heat gun. I tried the cellophane bags with the heat sealer and found it just too cumbersome. I lay the bar face-down and gather the plastic wrap around the back at the center and cut away the excess. Then I use a heat gun on the lowest setting to shrink-wrap. Then I stick a label on the back.
You do have to keep the heat gun moving at all times. Sweep it across the back a couple of times, make a couple of quick passes on the sides, and then briefly hit the top. And use the lowest setting. Have you watched this video on Soap Queen TV?:
http://www.youtube.com/user/soapqueentv ... D4bJgtaHMgKeep in mind that you do need to wrap melt-and-pour soap in some sort of plastic wrap to keep it from sweating or drying out. Then you can use a cigar band or raffia or wrapping paper, etc. Organza bags are nice, too. You could also use a cellophane bag and tie it off with some raffia. I'd probably still wrap the bar in plastic wrap first because I've had mine sweat or get the cellophane bag all gummy if I didn't wrap the soap first.
Hope that helps! Good luck!