Another definate DO is tinning the wire before inserting it.

Scale only has it right, the solder will wick in to some very tight places, even against the pull of gravity because capillary action will cause it to follow heat and flux. If you already have solder on the wire when you insert it into the coupler, the solder will also naturally flow into the socket. You also have the reassurance of KNOWING that solder is in there properly.

Clean (sand) and flux the tip of the wire about 1/8" longer than the depth on the coupler. Heat the wire up and watch the solder flow between all the stands. If you put too much on, you might have to sand the excess off to get the end into the socket, but it will go in and as I said, the final solder will flow easily into the socket.

The best solder to use is the acid core stuff from Radio Shack. It has a heat activated acid flux in the solder itself and that will help to clean any nasty stuff that might still be in the socket.

Ed