One of the easiest ways to perk up a beer ingredient kit is to use a better yeast. Yeast plays a big role in how the beer will taste and many times you can improve the quality of the beer you brew by substituting a different yeast for the packet the kit maker supplied.
But there is also the question of, 'what yeast should I use for this type of beer.' A lot comes down to trial and error, which is half the fun of home brewing.
Here are some articles about yeast:
Liquid/Dry Yeast Chart by Style - remember these are subjective, but they give you a starting point and you can experiment from there. Another good resource is ask the staff at your local home brew store, they generally have a lot of knowledge and experience with different yeasts.
Liquid to Dry Yeast Conversion Chart - you can save some money if you use dry yeasts.
Homebrew Yeast Strains Chart
Pitching Rates for Fresh Yeast
Yeast Strain Sources
Another reason for selecting a different yeast is the temperature that you are going to ferment your wort at. For instance sometimes it is hard to keep a basement warm enough in the winter for the yeast that came with the kit. But other yeast strains may be better for Summer or Winter brewing because they work cooler or warmer than the kit yeast.