Normally you can grab hold of the body of a DC jack to tighten the nut on the outside of the panel. However, I was making a small LED lamp where I couldn't hold the jack's body, so I made a tool to grip the inside of the jack (from the outside) to allow the nut to be tightened. The principle is the same as the tool for gripping threaded retaining rings. A shaft is turned down to a neat fit inside the jack, split, and a screw threaded through one half to allow them to be spread apart. This grips the jack very securely from the outside.
Video of operation:
I must add a link to a similar little tool called "Jack the Gripper" which is used for holding ¼" audio jacks (electric guitars etc.) for tightening. The author's website is http://www.frets.com/HomeShopTech/Products/JackTheGripper/jackthegripper.html and they are stocked by the likes of LMII. It uses a clever off-axis knurled cam to bite into the inside of the jack as it's rotated.