To start off, Kellogg's has a great Fuel for School rebate deal going on - buy any 10 Kellogg's items (includes Eggos, MorningStar Farms Veggie Foods, 100 Calorie Right Bites, and more) and get a $10 Rebate Check back from them - as well as a $70 off coupon from Dell. Sign up for an "All K Pass" here: http://www2.kelloggs.com/FamilyFocus/contentcta.aspx?id=1320
Next, Kellogg's has another promotion going on - They've partnered with Feeding America, which is a non-profit organization that is striving to feed the hungry in America. Right now if you go to their webpage and donate at least $5 to Feeding America, you will instantly earn at least $5 worth of Kellogg's coupons. (When donating, you go to the "Other" amount field, and fill in $5) Here's where you find that: http://www2.kelloggs.com/General.aspx?ID=1978 - It appears as if you always receive $1.50 off one package of Frosted Mini Wheats Little Bites, Jumbo Multi-Grain Krispies, or Corn Flakes, $1.50 off two packages of Special K, and $2.00 off any three Kellogg's Cereals.
RedPlum.com currently has printable coupons on their site for $1.50 off any two packages of Frosted Mini-Wheats, All-Brain, or Raisin Bran Cereal. You can print this coupon twice.
Now, as far as the best time to use these - wait til they go on sale. I know Target just had Raisin Bran on sale for $1.88 a box, which ended up with people actually earning a profit after rebate. Meijer has a great deal going on with getting $10 off of a backpack (or a free backpack if priced less than $10) when purchasing Kelloggs, CVS has a BOGO sale going on with Kelloggs - Sales on cereal are really popular, so it shouldn't be hard to find. In the end, you'll end up paying less than $1 each for the boxes of cereal. (Plus, obviously, the $5 donation) You'll not only be supporting a great cause but saving tons of money!
As a bonus, Kellogg's also is offering free DVDs with tokens taken from the boxes - http://www.kelloggs.com/promotions/foxdvd/ so you could earn 1 free DVD and be really close to a second! Feel free to leave a comment and let me know about the great deals that you get!