Some people believe the worst thing you can say in your life is "If only I could live it over again". They would like you to believe that you should live your life so as to have no regrets. Personally I don't buy into this philosophy. We all have, and will have, regrets based on how our experiences shape us as individuals. It is a natural part of our spiritual growth. For example, I have plenty of regrets about things I've done in the past but at the same time I recognize that the life lessons I learned from these mistakes have made me the person I am today. Not having made that mistake or those choices wouldn't have taught me the corresponding lesson, and as such I would not be the person I am today. This is why the idea that you could go back and relive your life is so dangerous. If you didn't take with you the memories of what you felt went "wrong" the first time you would just end up making the same mistakes again.
Not all mistakes fall into the above category obviously. I remember one drunken night thinking it would be a good idea to hang out the window of a school bus driving down the 401 Highway so I could shake the hand of a guy driving in a car next to the bus. Pretty @#$%ing stupid and I'm pretty sure I didn't learn anything from the experience other then I was lucky to be alive and lucky that the guy holding onto my legs wasn't as drunk as I was.
The Do Over Day website has a bulletin board where you can post your regret stories, send e-cards, upload videos and generally connect with others.
Realistically one of my bigger regrets is not taking University more seriously. Not that I have some dream about that translating into having a better job or more money today. For all I know I would end up in the exact same career. When I think back I just remember being lucky enough to have had some very talented Professors, and I guess I regret just not actually showing up to class in order to interact with them more often.