My kids usually eat lunch in the cafeteria at school, but some days the lunch is just icky and I let them take their lunch to school.
At night I prepare the dry part of the lunch (chips, cookies, etc.) by placing small amounts in snack size plastic baggies (I have to buy name brand plastic baggies for food because the store brand ones make your food stink, or to me they do.) I put that and a piece of fruit in their lunch bags (got some nice basic ones at Target for $3.99 each) at night before I go to bed. In the morning they usually like to make their own PB&J sandwiches, and then I cut them up into halves or fourths and put them in sandwich size plastic baggies. The last thing that goes in is a juice box or water bottle. (I haven't had either this week so they've gotten milk from school.)
The hardest thing with school lunches is getting the girls to remember to bring their lunch containers home!