K.B.
Books, like Dr. Suess' ABC and Chicka Chicka ABC.
Magnet letters for the fridge and foam letters for the bath tub.
I also got in the habit of singing it while brushing DD's teeth - it's how you know you have brushed long enough. So she was also hearing it twice a day.
I am not anti-TV - a little Sesame Street or Super Why goes a long way (both are on PBS).
DD could recite the whole alphabet by age 2.