Try Valtrex. It's prescribed for herpes simplex 2 (genital) but also works for simplex 1 (oral). I have a prescription for when I get cold sores.
At the first sign of tingling/bump on your lip, take 2 pills. 12 hours later, take 2 more.
I use this in conjunction with Abreva, again at the first sign of a cold sore, and my cold sores never last more than 2 days. In fact, on Christmas Eve I woke up with one (UGH!) and now it's gone. My cold sores used to last a week or more.
The last thing I do is an old wive's tale but it also works. On the first day, get a Q-tip and dip it into acetone nail polish remover. Put the Q-tip on the cold sore for 10 minutes. It stings like I-don't-know-what, but it really dries it up and makes it disappear!
I know I sound like a commercial but I swear by this method!