I use a purple disappearing marker. I find it tends to stay for several hours unless it gets sunlight on it, in which case, yeah, it'll just vanish straight away. :)
I've used the blue water-erasable markers but have read from several people on this board that the lines reappeared after a year or so and then were brown and indelible. Ouch! I haven't used mine for long enough to know whether that's actually true (sometimes people will just repeat something they've heard!), but it's making me a bit wary of them.