More fermentable ingredients... When you say 'very high alcohol content' what is your quest? Just be sure to balance the high ABV with a good malt backbone and enough hops to not make a sugar junky scream in bliss.
Look at recipes for things like RIS, different imperials, DIPA's, etc... Robust porters are also good to look over. Depending on the yeast strain, it could attenuate to the point where it finishes too dry and it's not something good to drink. Generally, as long as it's not TOO dry, aging will take care of that. But, if you're going to bottle this, you could be looking at an extended bottle carbonating time frame too.
Making a beer with high ABV that's great to drink, it neither simple, easy, or fast. It's why there are so few of them, generally, available.