I've stumbled upon an interesting concept with the little ones. If I just spend about 30 minutes or so doing an activity with them, they are more likely to keep themselves occupied while I am schooling with the older kids. This year is a little different, because my older two are doing more independent work, and I am spending more time teaching the younger ones this year, but this has really helped me since I figured it out. :)
You really could spend the 30 minutes (or more if time and attention allows) doing just about anything. What really worked for us was reading a book and doing a simple activity based on that book. For example, you could read "Blueberries for Sal" and then make some blueberry muffins. Or, make a color book with construction paper and magazine cutouts-one color per page. There are tons of possibilities here and you don't even have to get complicated. Read a Bible story and have them do a color page. The key is the time spent.
I've also found websites that have lessons for the "letter of the week" or the "number of the week" or the "theme of the week" or whatever. These are great because they are learning basics and may make it easier for your organizing and planning. You can plan by week instead of coming up with something totally different each day.
I contributed to a Preschooler E-book for the Old Schoolhouse Magazine that is scheduled to come out in March. I'll keep you updated. There will be some great information in there for Preschoolers.
Here are some links that I have found to be helpful:
http://www.alphabet-soup.net/ (preschool lessons, printables) felt stories
http://www.starfall.com/ (some writing printables)
http://www.dltk-kids.com/ (pre-K Bible color pgs., abc & 123 templates, crafts, recipes, felt)
http://www.shirleys-preschool-activities.com/ (tons of preschool)
http://www.letteroftheweek.com/ (preschool activities)
http://w3.gorge.net/cannon/children.html (preschool resources)