You cannot have any foreign sustance on the disc, like blinker lights or weighted tape.
sharpie or magic marker is ok, because you need to have your name on your discs.
In tournements, you should have a mini disc for marking your throw. In casual play, sometimes people will flip their disc over on the ground, but you can't do this in tournement play. If you don't have a mini, you can probably get one when you sign in, or just ask someone if you can borrow one. People generally have extra minis they can loan you.
And one last thing. Bring an extra pair of socks, and shoes if you have them. No one seems to mention this, but I find it really helps to put on dry socks between rounds. Playing all day in sweaty wet shoes is a real drag.