I agree with many of the kindergarten teachers that have posted of how crucially important it is to use number stories from the beginning of the school year to help with the concept of addition and subtraction. I have used the resources Math Their Way, Developing Number Concept with Unifix Cubes and Explorations ( an out of print Addison-Wesley product) to develop my math program for years. Math series come and go in my school system, but I've never found any thing better than the above mentioned resources. Work Jobs II has many story mats.
Below are a few of the things you can find on my website.
Go to Math Literature on Linda's Learning Links for more ideas!