Tip number 1 for eating frugally: Drink only water.
Filtered water is best. For just $20 you can buy a new cartridge for your water purification pitcher. That will last two months. The pitchers themselves come to anywhere between $40 and $50. It sure beats buying the bottled kind!
If you must buy other drinks, drink frozen orange juice or homemade lemonade. Reconstituted nonfat dry milk is best though. It provides calcium and other nutrients without the fat and has more bang per buck than any other drink than water.
Giving up caffeine will be hard for me. Starting tomorrow, I will only drink water and sometimes a little nonfat dry milk on my oatmeal.