To give up food, TV, or -gasp- the Internet, is helpful to show us that God is god and not food, TV or the Internet. If I rely on anything more than on God, I should think about doing without this thing for a period of time. One way to decide what may be the most important object of fasting would be to decide what is the thing you think you cannot do without. Then decide to do without it for a given period. One of the first lessons I learned about fasting from food was that I should have a specific matter to pray about, then, every time I felt hungry from not eating, I should pray about that request.