I went to Mosport on August 26, 2007 for the American Le Mans race.
Here are some tips:
1) Enter at Gate 4. There are many places to get into the track but this one will put you at the starting line. You can buy your ticket for cash when you enter. Programs are available there too.
2) Bring cash. You will need it for food and drink and any souvenirs. But I only spent $50 for my ticket, $10 for the program, $8.50 for lunch and $2 for a Diet Coke.
3) Bring a FM radio with fresh batteries. The PA is broadcast on 90.7 but my batteries were dead, so my radio was useless. It gives you the play by play which is necessary to follow things.
4) Print off the schedule and drivers list at home. That way you won't need to care the program around. You will need to refer to the schedule often as a lot happens and you need to plan your day.
5) Print off a map so you can find your way around. There are few stands so most people wander around to find a place to watch the races. I think it is best to do this even if I prefer Moss Corner.
6) Program the scanner at home and charge the battery. The battery only lasted through the starting lap of the big race. I missed it.
7) Bring Red Bull. It gives you wings and they support racing.