Here's a few helpful tips. Decide before the cards arrive how you are going to conduct the fundraiser. Have the system in place to easily keep track of the cards and money. You might want to work in smaller teams. Perhaps ten to fifteen on a team with a parent as team leader.
Smaller groups are easier to monitor and easier to motivate.
You can get some competition going between the teams and the individuals. This can greatly increase your sales. Take a Saturday and set up a table at your local supermarket, Walmart, or any other high traffic establishment in your area. Show the cards to everyone coming in. Permission to do this is usually easy to obtain.