Check out the BJCP Exam Study Guide on the BJCP website. It has a great guide for doctoring beers. I've tried to cut and paste it below, but it didn't come over so great.
Guidelines for Doctoring Beers
Flavor Adulterant Quantity
Sour/Acidic USP lactic acid 0.4 ml (1/3. tsp of solution of 1/8 tsp lactic acid plus 3/8 tsp distilled water)
Sour/Acidic White wine vinegar 3/4 tsp
Bitterness iso-hop extract 1 or 2 drops, to taste
Sweetness sucrose (table sugar) 1/4 tsp dissolved in 1/2 tsp water
Astringency Grape tannin 2 tsp. of solution of 1/8 tsp tannin dissolved in 5 Tbsp water
Phenolic Chloroseptic 0.4 ml (1/3. tsp of solution of 1/8 tsp Chloroseptic plus 3/8 tsp distilled water)
Clovelike Clove solution Make solution of 8 cloves soaked in 3 oz. of beer and add liquid to taste (about 4 tsp)
Sulfitic Potassium metabisulfite* Make solution of one tablet dissolved in 3 oz. of beer and add to taste (about 1/2 tsp)
Alcoholic Ethanol 2 tsp (increases alcohol by 2.7%). 3 tsp vodka may also be used
Sherry-like Dry sherry 3/4 tsp
Nutty Almond extract 0.1 ml (1/8 tsp of solution consisting of 1/8 tsp. almond extract plus 5/8 tsp. distilled water)
Papery/Stale N/A Open bottles to air, reseal, and keep at 100 F or warmer for several days
Winey White wine 2 Tbsp
Diacetyl Butter extract 4-5 drops
Estery Banana extract 6-7 drops
Lightstruck N/A Expose commercial beer in green bottles to sunlight for 1-3 days.
* Should not be tasted by persons with asthma or sulfite allergies.
Beer is good for anything from hot dogs to heartache.
Drinking Frog Brewery, est. 1993