Agree with tonyted, buy a new one. I was always told that the superstition stems from days when it would take a maid-servant at least 7 years to save up and pay for a breakage in their masters house and thus replace it - therefore ending the "curse". I have no idea whether this is true or not, but it really is just a superstition.
Octavius is correct. The superstition is not 7 year's 'curse' but 7 year's bad luck. Bad luck cos if you broke one of your master's mirrors you'd be paying it off for the next 7 years. The 'bad luck' ends when you replace the mirror (nothing about burying it in the garden).