It's the amount of fermentable sugars made available by the grains you add to the wort and how much of them the yeast can convert to alcohol (EtOH) before the concentration can rise high enough to inhibit further conversion of sugar to EtOH. I guess the short answer is anything (malted grains, malt extract, flaked oats / rice, etc...) you can add that provides sugars that your yeast considers tasty! Try the link below for better gouge.
There aren't really any tricks to upping the ABV without adding more ingredients. The yeast need the sugar to eat. I suppose you could use less water and make a smaller batch using the same ingredients. I don't even worry about ABV% as long as it tastes good.
Corn sugar can be added to raise abv% but will sacrifice body, head retention, mouth feel, etc. So basically, is the extra abv% worth the cost of a lesser beer? If your looking to get "krunked" up, make apfelwein :rockin:
Ha ha ha I'll second the Apfelwein comment, mine will probably ferment out to a little over 6% ABV without having added any fermentables at all.
On the topic of beer, a more experience member of the forums provided some helpful information. If you're tampering with pre-hoped canned kits just remember that adding more fermentables to it could alter the intended balance of ingredients in the kit and you probably won't get the best tasting product possible.