I tend to do two types of acclimation. The first is similar to Sk8tr since, in reality, salinity and temperature are the only really critical factors. The second is with local fish and any where the salinity is off by 3-4 points.
For that I open the bag, dump it into a container 3-4 times the size of the bag and immediately add about 1/4 the bag amount of my own water. Every ten minutes or so, I dump about 1/4 of the water out of the container and add that much of my water. After a few changes I'll check salinity and temp, usually pretty close, and, if needed, float the container for 15 minutes or so to get the temperature correct. Then I put the fish/invert in the tank (no container water) or dip the coral and put it in. This doesn't work if the SG is under 1.020 and I'm at my normal 1.025/6, then I just have to acclimate longer and hope for the best.
I haven't lost any live stock this way that didn't come in with symptoms of something, cold shipping, delay, etc. FWIW, the places I order from will tell me the tank conditions and I can match my QT tank to that before the stock arrives. I have one LFS that keeps fish AND corals at about 1.019, I just don't buy live stock from them.
[EDIT] My rule is: If I buy from a place three times and whatever I buy dies all three times, I don't shop there again. [/EDIT]
Jeff