The most comon mistake that home owners do when they terminate sub panels or remote panels is they bond the neutral bar to the panelboard. That means they put the green ground screw through the neutral buss "grounding" it to the equipment ground. YOU MUST NOT DO THIS. services require a single point of bonding and that has already been done at your main service panel. All subsequent feeders that go off to other panels, sub panels, or even recepticles must keep separate the ground and neutral wires. You may inadvertantly send fault current in to your house, or out to your subpanel/outlets creating a hazardous condition.
You must use 4 conductor wire (2 hots, Neutral, AND ground) appropriately sized for the amount of over current protection you are providing.
They now have 240V/ 50A RV plugs to create a lot of portable sub panel capabilities.
Above all, be safe. When in doubt, ask a professional. 99% of the time, they will help and get hooked on the blinky blinky too.....I know, that's how I got hooked.