One thing that is very common is reading programs for kids in school. This is a great idea, as they get prizes for keeping track of all the reading they do. Well, preschoolers are no different in this area, and many would argue that they need just as much reading as other kids, so why not do a program for your preschooler?
For each book you read you can mark it down on a chart. There are ways to make this fun too. You can make each day be a different book. This would cause some awesome trips to the library. You could let the kids use a sticker for each day to mark it off (might cause them to be happier about reading and encouraging you to read with them). Don’t worry though most little kids love reading.
At the end of the month you can take them out for ice cream or pizza, and make them feel special for completing their month of reading. Maybe make them a bookmark for next months reading, something like 1 month of reading and going strong. Let them decorate by coloring the bookmark and putting stickers on it.