Where to ask for food
Reach out to local farmers' markets, grocers, or restaurants. OccupyDC has had some luck with asking individual vendors at the Dupont Circle farmer's market and local businesses for unsold goods at close of business.
You may also be able to use donated money to score discounts on group purchases of some staples or equipment at restaurant supply stores.
Sanitation is mandatory, especially when handling meat, dairy, eggs etc... (think animal matter); clean all knives and preparation surfaces in between every stage of preparation. [BEWARE OF CROSS CONTAMINATION]
Hand Wash Station
Setup two containers adjacent to each other. In the first put soap and water, in the second put one half cap full of bleach per gallon. Make this your hand wash station. Inform everyone upon entering the kitchen area that they should wash their hands in the soapy water and rinse in the bleach water. Encourage everyone to do this often, but especially before handling or preparing food.
The bleach will make everyone's hands raw, but it is worth it not to poison other occupiers.
Set aside a place for trash several meters from the food prep area. Let everyone know that all trash goes there immediately. it's not appropriate to store trash anywhere near food items. At intervals have a volunteer sort recyclables out of this heap and bag the garbage for removal.
Compost may be a great ideal but it will attract masses of insects to your camp, only really useful if you want to plant a garden, and that is only possible and useful if this movement can hold its ground until after the winter.
Clean your tables / surfaces
Secure some rags and soak them in a mild beach solution (one cap/gallon). Use these rags to wipe down your kitchen surfaces as often as is practical. This will prevent cross-contamination and discourage the ants from raiding your picnic.
Food should not be stored on the ground. At least store the food on a tarp, but wooden pallets (or similar) that can get the food a few inches or more off the ground are much prefered.
What to bring
Non perishable items are preferable, beware of home-preserves, if improperly prepared they can induce botulism.
Give thanks before each meal, out of solidarity for those who don't have meals. [It is also important in maintaining both perspective and morale]