You can use more extract, but then you'll need to adjust your hops to keep it balanced. You can add a some honey or corn sugar, no more than a pound in 5 gallons and even that is pushing it. Whatever you do, don't use table sugar.
My personal favorite way to boost the alcohol is to have a glass of single malt scotch while enjoying a homebrew
Depending on the type of beer you're brewing, upping the abv could very well take the beer out of style and make it taste kinda nasty.