Keep It Clean
For the fastest production, take the time to keep your soldering materials clean. Even a dust-sized speck of dirt can cause a 3/8 inch open crater in a solder joint between a pipe and a fitting. A piece of brush bristle caught in the flux can easily cause a joint to leak.
One of the most important soldering tools is a clean dry rag. After sanding the copper pipe end and the inside of the copper fitting, wipe the pipe and fitting clean. A quick but thorough wipe is all that is needed to be sure to get the abrasives and sanding debris off.
Flux should be clean! Don't let junk get in the flux. If anything falls into the flux tin, skim the top of the flux with a piece of cardboard and discard contaminated flux. Don't touch your flux brush to a hot pipe and then toss it back in the flux can - the burned brush bristles will fall off in the flux.
A little precaution can save a lot of lost time remaking joints.