Play games of "I Spy" at the store or at home. Have your preschooler find a certain letter on labels, signs, books, etc.
Make writing utensils and paper available to your preschooler. Encourage your preschooler to write letters by making books and pictures with words.
Keep magnet letters on the refrigerator.
Play music and "dance" by forming letters with your body. See what letters your preschooler can make.