Use lemon zest. Boil it for 5 min as stated above. To zest it, either use a zester, or something like a cheese grater, and just rub the lemon on it to take the yellow part of the peel off. Use the freshest lemons you can, and for a nice hint, use about 1 lemon per 2 gallons. (for light summery beers. <1.035 SG) Again stronger the beer the more lemon you need.