I don't remember if it was last year or the year before but I had one of the small Dakine cable locks. It was nice because it was compact and fit in my pocket. However, one day I couldn't get it to unlock while my board was attached to a rack. I was able to pry the plastic body open on the rack itself and get my board free. Surprisingly, nobody even questioned me while I did it.
My point in bringing this up is this: If you're going to invest in a theft deterrent, I would avoid the plastic housing style locks. I had my board free in under a minute.
Going with a self-coiling style lock like what Rider161 posted above would be choice, even if it is slightly heavier to carry around.
i bought a plastic FLOW combination lock from a local place because it was cheap. and that's all it was, cheap shit. when i got home and was "practicing" with it locking my board to shit, it wouldn't unlock, i tried everything and ended up destroying it with a screw driver. the POS didn't last 5 minutes and i could imagine getting pissed off from not being able to get it off the rack on the mountain.
go get a bike lock that uses a key and if its heavy just strap it to the rack at the beginning of the day.
Do like others have said and get a heavy duty lock and just lock it to a pole or fence in the morning, just make sure you don't put it in a really busy spot because there might be boards all over your pot when you go to lock it. I still can't believe how many nice boards i see unlocked, I cant complain though because it just makes it easier for me to trade up(joke)