Keep your camera in your purse or car. I have found that if I have my camera with me all the time, I am less likely to miss some great shots. I get frustrated when I don't have my camera on hand especially when others have them with them. I have caught the best memories by having my camera in my purse or bag!
Make sure you keep your digital camera cards clear or an extra roll of film in your camera so you will not miss those passing moments. Also make sure your battery is charged or you have an extra battery for your camera in your camera bag.
All is not lost if you forget your camera. Ask if you can have the doubles of others' pictures or ask if you can bring over a CD to burn a copy of what others have taken. Even though I bring my camera to an event, I will sometimes ask if I can have another person's pictures as well. I will exchange what I have taken with them.