I second fermentation temperature control. Try to pitch your yeast at your desired fermentation temperature and control that temperature throughout fermentation. When I started controlling my fermentation temperature, it tasted much more like commercial craft beer. Fermenting too warm can cause an excess of esters from the yeast.
If you can't do full boils, use late extract addition. Some argue that doing a concentrated boil of extract causes "extract twang" from the Malliard reactions. Add enough extract to reach a gravity of about 1.040 (about 3-4 lbs extract for 3 gallons water) in the wort, then the rest towards the end of the boil. Make sure to adjust your hops, as you will get more utilization out of them with the lower gravity boil (another good reason to use late addition: can use fewer hops). Search this forum for "late extract addition." There are many threads with a lot of good information.
Use fresh ingredients. Liquid malt extract can go stale if on the shelf for a while. If you can't determine the age of the liquid malt extract from your local homebrew store, then use dry malt extract. It stays better much longer.
Pitching proper amounts of yeast is also seconded. Make sure to rehydrate if you're going with dry yeast.
You can definitely make award winning beer with extract. I've never entered any competitions, but some of the extract beers I made were very good. I was actually surprised to find out how many award winning extract beers there were when reading "Designing Great Beers." In some categories, half of the NHC second round competitors were just extract+grain brewers. Making good beer is more than extract versus all grain, it has to do with sound brewing practice. Proper sanitation and proper fermentation control have much more to do with brewing award winning beer.
Edit:
Also, if you're not already doing so, use only light malt extract and get flavor and color from steeping grains. The only exceptions to this are special extracts required to make certain beers, i.e. Munich malt extract, Wheat extract.