Take a soluble aspirin, run it under the tap until it starts to fizz, and then press it against your cold sore. You'll feel it tingle as it comes into contact. I'm not entirely sure how it works, but I think some of the ingredients in aspirin are the same as those in coldsore medication. After a minute, remove the aspirin. There will be a white crust on the coldsore where the some of the aspirin has stuck. Just leave this for as long as you can (it helps if you don't have to leave the house!) When you want to remove the white crust, make a salt-solution (cooled boiled water and plenty of salt). Then take a cotton ball, dip it in the solution and hold it to the coldsore until the white crust dissolves.
This method keeps the coldsore dry, stops it itching, and reduces the amount of time you're left with a blister.
To avoid coldsores in the future, use Elizabeth Arden 8 Hour Cream as a lip-balm every night before you go to bed. If you get coldsores on the outside of your lip line, smear a little extra around your lips. 8 Hour Cream was originally designed as a medical balm for cuts and burns, so perhaps that's why it works so well.
Personal experience. I've been getting coldsores for years and have tried everything. I haven't had an outbreak in the 9 months since I started using Elizabeth Arden 8 Hour Cream as a lip balm at night.