I find that it really helps if you eat a lot of fruit and vegies and take a multivitamin. I was having chips or toasted cheese sandwiches for lunch/dinner (I work afternoon shift) and lots of bread and was feeling so tired.
Now I remember to take my multivitamins every day, have salads and at least 2 large pieces of fruit, heaps of water and I feel heaps better. Low and behold if I go and get a snitzel pack or something the next day I am exhausted!