Fasting is a great way to break through many struggles and draw near to the Heavenly Father. No-one has to go completely cold turkey and jump right into water alone, though many may argue against that. The fact that your giving up on food for the Holy Spirit to speak to you is enough. The Father respects that and answers us when we do that.
The number 1 thing to look out for are the people close to our lives! A friend coming by munching on a bag of chips, offers one and you quickly eat it. You'll quickly realize the change from the spiritual state you were just sitting in before you swallowed. For some reason, it seems, there are a lot of these moments that occur near the start of a fast for me.
The number 2 thing, is just as important as the number 1! Whatever you drink, make sure it's pure as you can get it. Tap water will not cut it in most places. The chemicals they add in our water supplies actually shut down the portions of our brain that connect to our spirit! Fluoride is the worst one, followed by chlorine.
Everything else after that is pretty cut and dry. The closer you draw to the Holy Spirit, the easier it is to see things in your life you need to be aware of. There is darkness somewhere in all of our lives, but with my fasts it always seems the shadows grow just before breaking through to Him. Typically this happens on the second day of fasting but the breakthrough has come on day 3 or 4 as well.
It's really fun to learn to operate your body through willpower alone. Eventually in the fast, if it's long enough, you'll get a feel for where your spirit within you exists, as your flesh loses it's ability to dominate the mind.
I've smoked through fasts, but on the American Spirit ultra light packs - the white and yellow boxes. I get very sick if I'm fasting and smoke anything else.
You may 'hallucinate', but in corporate fasts we've always seen the same things at the same times. Respect the Light and tell the shadows to leave, if you 'hallucinate'. Your words will have seemingly strengthened power over things around you, if spoken from peace, love, or humility.

Hope this helps.