I don't see this in anyone else's answers so I'll add...
As my mother always reminded me... "Keep your tip clean!"
This is a mistake often made by the inexperienced. Failure to do so will result in a build up of solder on the tip which has burnt off the flux and becomes oxidized. This residue will be deposited in your next joint making it weak.
Whenever soldering you should have, as part of your toolset, a wetted sponge which you should use frequently, even before and after every joint, to clean the tip of the soldering iron. Simply draw the tip towards you across it while rotating the iron a few times till the tip is clean and shiny again with no excess solder build up on it.
The hissing noise when you do it is also quite emotionally rewarding.
Note if the sponge is very wet, you may need to pause to allow the tip to regain the appropriate soldering temperature again, but with most modern irons, that does not take long. If you are doing multiple joints at once, you may get away with less frequent cleanings, but you should get into the habit of cleaning the tip frequently, and definitely if you pause for any significant duration.
BTW: There is an alternative method that uses copper coils instead of a wetted sponge which reportedly works better though I have not tried it, and have my doubts. It also just seems like a waste of a rather valuable material to me.
Also, as I mentioned elsewhere... always use heat-shrink tubing on joints like this. It adds strain-relief which will help prevent the wire from breaking at the solder interfaces and protects you from stray broken wire strands shorting to the next pin.