What other fish do you have? You know angels are prone to nipping corals (unless you're lucky enough to get a nice one), right?
Start off with a reputable LFS. Make sure the fish is eating at the store (a good LFS doesn't usually refuse a feeding). When you get them home, do a drip acclimation for 1-2 hours (the longer the better).
Feed the fish in your tank while you're drip-acclimating, too.
Before adding, acclimate the bag's water temp. Then, turn off the lights, and add them in (don't get the LFS water in your tank).
If you're planning on QT'ing them, same thing -- drip acclimate to the qt tank water, them float the bag to equalize the temp. Make sure they have a hiding spot.
The rest is just hoping your fish is healthy enough to handle the move :)