Hmmmm, well i dont know a lot about fasting except you take in a lot of water.... but, if you do that I would recommend you take a multi-vitamin (there's one you can dissolve in your water called eco-boost? I get it at Costco)..
I dont know if that helps
Judaism 101: Tips for Yom Kippur Fasting - I found this link about fasting in Judaic culture.. maybe it'll give good tips