I started preschool with my oldest son, who is now in 2nd grade and saved all of the activities so they're ready to go for my next two preschoolers. We have added lots of new activities, but it definitely makes it easier the 2nd and 3rd time around. We do one letter each week and choose a theme for each day starting with that letter.
Something else I've learned is to READ, READ, READ with your kids. They love to sit with a stack of books and have us read to them ... or grab a book and sit down somewhere and "read" to themselves. It's so fun to watch.