Sounds like you're giving the joint way too much heat for too long.

Plugging a spare connector into the one you are soldering keeps the pins from warping and help make the plugs easier to unplug in the future.

The tip about a clean tip and flux is very important. If you don't use flux, be sure to use solder with flux core.

With a larger guage connection like 10 or 12 guage wire to deans it helps to use the tip of the soldering iron to apply heat to both the connector and wire at the same time.

With practice you'll get the hang of it in no time.