Add more crystal/caramel malt. Use more than one "L" rating. So for example use 1/4 lb of C40, 1/4 lb of C80 and 1/4 lb of Special B. You could add more, but by the time you get to a full pound of crystal malt, the beer gets a little too sweet, IMO, especially with extract based beers since the extract already has a good portion of crystal malt in it. If you really are looking for something sweeter, go ahead and add more, go up to a pound and a half - why not? I don't care!
Also, instead of using specific "L" rated crystal malts, you can use the "light - medium - dark" crystal malts that the British maltsters make. So in that instance you might use 6 oz of each of those, or 1/2 lb each of the light and dark... experiment!