1. Do a full boil.  Get a larger pot (7+ gallons) and a wort chiller and boil your full batch size.  That way you won't have to deal with the over-caramelization of a high gravity boil.

2. Add 1/3-1/2 of your extract in right as you are coming up on boil, then VERY SLOWLY add the remainder during the last 15-20 minutes of the boil.  This will reduce the net amount of time for caramelization which will reduce that "extracty" flavor.  A couple of cautions: you need to add the extract slowly and stir well to prevent scortching of the extract as you add it, and also, adding it slowly will allow you to maintain a boil and continue your hop extraction.  If you add too much extract quickly, the temperature drop will kill the boil.

While the first technique is recommended, the second is a little more practical for most people.