A lot of you seem to be using the words "trickle charger". A trickle charger will produce a low current 12v charge, but will continue to maintain that same charge rate regardless of the charge state of the battery. This is not a good thing to do.
What I believe most are meaning to say is "battery tender" of sorts. These charger will not charge a dead battery, but will maintain proper voltage in a charged battery. They have a brain of sorts that will allow the charge to go off when the battery voltage has been reached, but kicks back on when it get below a predetermined voltage.
There are many brands of these available, I use one on my motorcycle all the time. Like TomS said, wire up a two connector plug that has a rubber snap cap that covers the pins when not in use. Then wire your battery tender with the mating plug.
These are available at most motorcycle shops for just a couple dollars.
