If you are soldering onto an XT60 and the pins start to sag out of alignment, here are some tips.
Use a big enough soldering iron so the entire soldering job can be done quickly. I use a 100W soldering iron with a fat chisel tip, which provides lots of thermal mass, so the iron doesn't cool down as soon as it touches the subject.
Insert the male and female connectors together, and don't separate them until the soldering is done and the plastic has cooled down.