Originally Posted by Dave H

Most common mistakes:

Not using flux, doesn't take much (or using the wrong kind, plumbers etc).
Not pretinning the tab and wire with some solder. A little flux here too doesn't hurt.
Also it is very important to not let the connection move after soldering as it hardens. Don’t blow on it either, let it cool naturally.
Possibly wrong solder or too old solder. (again, this ain't a plumbing job)
For long term, avoid corrosion, be sure to clean the flux off (somewhat dependent on the exact type of flux).

There is a wealth of solid info on the sticky at the top of the page of this section, much more complete than the video (as I remember it from some time ago).