Familiarize your preschoolers with the looks of the letters. Show them flash cards or draw the letters out on a board. Tell them the name of the letter, as well as the sound(s) it makes. Repeat yourself several times and ask them to say it aloud after you, to make sure they're getting it before you move onto another letter.
Give your preschoolers alphabet printouts with letters for them to trace and pictures demonstrating the letter's use. These can be found in books or online. Trace the letter with the child if they're having a difficult time.
Sing alphabet songs. The classic alphabet song that we all learned as kids is a great start, but there are lots of other songs available to sing along with on the Internet. Catchy tunes are a great way to get ideas to repeat in kids' heads.
Play matching games where the children must match the lower case letter with the upper case letter. Draw the letters on large pieces of paper and have them laminated. Give every child a letter and pick a random upper case letter from the pile. Ask the child with the matching lower case letter to come to the front of the class when they see their matching letter.
Have your preschoolers play alphabet computer games. There are many games available online for free that are simple and educational. Computer games also give the preschooler a chance to interact constantly, as opposed to being in a classroom setting where they may have to wait awhile for their turn to speak or interact.