I have found that one of the best ways of incorporating beer into cocktails is to use it in place of champagne or soda water. By utilizing beer in this way, you open up a simple cocktail to all sorts of variations. If you were to add a nice Belgian ale, you would be gaining some sweet malt and yeast flavors to your cocktail, but if you used an uber hopped IPA instead, you would add a fantastic bitter/citrus dimension to your drink.
The thing to keep in mind when using beer is balance. Some beers, especially Belgians can be fairly sweet, so make sure that you are adjusting your cocktail accordingly. This next drink pairs one of my favorite spirits with one of my favorite styles of beer.