1. Get at least 8 hours of sleep every day. Go to bed at a reasonable hour, and get up when you wake up.
2. Get a small notebook and a pen. Carry them with you everywhere.
3. Eat properly. No excessive junk-food. Don't skip meals.
4. Ever day for at least 1 hour, go to a quiet place and bring out your notebook. Spend the entire hour writing out ideas. It doesn't matter if they are good or bad. Just write them out. Do it. The point is to use your "idea muscles".
5. Spend at least 1 hour every day walking, running, moving, or doing something to make your body active and actually get your blood pumping. Even if it's just 5 minutes here, and 2 minutes there... do that a bunch of times every day.
6. Repeat step #4 at least two or three times per day.
With the above steps, you will condition your body and brain to be a thinking machine. You'll find after a week or so, it will become easier and easier to think up ideas for your notebook. Soon you'll have hundreds of ideas written down. Eventually you'll get an idea for a new startup that just work. Pursue it! (Don't stop the steps above, though... maybe just limit them a bit.)
Remember, your brain is a muscle. Exercise it. Keep your body in decent condition because it feeds your brain.