What, if anything, should you do differently when brewing a high-gravity beer?
First, make an appropriately sized starter! Pitching a regular old vial into something like a 1.090 beer is a good way to overstress the yeast and get lots of undesirable esters, and have a beer that didn't finish out completely. Jamil has a helpful writeup on pitching rates and a pitching rate calculator that will help you size a starter.
Second, pick a yeast that is known to handle the higher alcohol.
Third, make sure your aeration step is solid. Your beer will need to be well aerated to ferment out completely.
By following these steps, you should be able to approach 20% ABV:
- As Morgan said, make sure you use enough yeast!
- Aerate the beer with pure oxygen at the start of each fermentation phase
- Add sugars and yeast during secondary (and/or tertiary) fermentation
- Use different yeasts for the different fermentation phases. Yeasts that have high alcohol tolerance don't necessarily like high gravity, and vice versa — start with a yeast that can take high gravity, finish it off with one with high alcohol tolerance.