Make letters a part of the child's environment. Hang banners and posters that show the alphabet at eye-level for children, so they become familiar with what letters look like from the time they are toddlers.
Teach children "The Alphabet Song." Music helps children memorize concepts in a way they'll enjoy. The repetition of singing the alphabet song every day will help children be able to learn the names of letters and their correct order from the time they are 2 years old. Play recordings of the alphabet song and sing it slowly to children at least once a day. Keep singing it regularly even after they have learned the words.
Once children have entered preschool, choose a "letter of the week" each week. Introduce the letter at the beginning of the week by posting a large picture of the letter in a main area that children can see. Talk about words that start with the letter and what sound the letter makes. Throughout the week, do related activities; do art projects and read books about objects that start with the letter.
Once children are familiar with what the letters of the alphabet are, they can begin to work on writing them. Use tracing worksheets like the ones that Scholastic has on its website. Give children worksheets that have letters printed in dashed lines that they can trace. This way they can practice forming the letter without having to be able to write it themselves.
Once children have had some practice with tracing the letters, let them use art materials to practice further. Let children write letters with finger paints on big pieces of paper or ask them to trace the letters with their fingers in the sandbox. If children can have fun with letters and make learning a hands-on process, they will be more engaged and excited about what they're doing.
Once children are comfortable with tracing the letters of the alphabet, let them practice writing the letters themselves. Write each letter at the top of a sheet of paper and have the child copy it 10 to 20 times. Check on their progress and make corrections if necessary. Repeat this process with each letter of the alphabet over the course of several weeks or months. Don't overwhelm children by asking them to do more than one or two letter sheets a day.