If you are sweating a pipe that is already in place against a joist, you might have to put something in front of the joist it to keep it from burning.  I usually keep a fire extinguisher handy if I am working in this situation.

You can't sweat a fitting on a pipe that is full of water.  The fitting will not get hot enough and if you just heat the solder the joint will leak..  If you are trying to sweat a fitting on a vertical pipe that is full of water, use your finger to force a couple of inches of water out of the pipe.  You might have to heat the fitting for awhile.  This is the worse-case scenario and can be trying.  If the pipe is a horizontal one that had water in it, try to bend it slightly to allow the rest of the water to run out of it.  Next force a rag into it to dry it out.

You can remove a soldering fitting by simply heating the joint.  Use a pair of pliers to work the fitting loose as the solder gets hot.  If you are going to sweat a new fitting on, sand as much of the old solder off as possible.  If you don't, the fitting might not fit.