What sort? I wouldnt recommend anything dairy milk based, like these powders you add to milk that supposedly replace a meal, as they're highly processed and contain additives
and all sorts of junk. Basically it depends on the reason for your fast too, if it's for detox then soda, coffee, milk etc want to be out, if it's purely for weight loss then you could 'manage' your input and still probably lose. Saccarin and artificial sweeteners present in diet drinks are pretty toxic and considered to be potential carcinogens so they're best avoided, especially during a fast. I'd say try and maximise your health and weightloss with a fast by including healthy juices, lots of water, herb teas, miso if you like it (in hot water) and of cause lemon is considered to be the king cleanser, read up on The Master Cleanse
too. All the info's out there, good luck Emma.