How do I prevent dehydration? I have vommitted twice today, what should I do to ensure I don't become dehydrated?

Clear liquids. Drink frequent sips of clear liquids, such as sport drinks or flat ginger ale. Monitor your urine output. If you have fewer than three voids in a day, or your urine looks very dark yellow, then you are dehydrated and may need to be seen in an urgent care or emergency room.
See below. Reason for vomiting should be figured out and that should be stopped. If you keep vomiting, you cannot get enough fluids by mouth and may need IV fluids if it becomes neccessary.
First . First off, why did you vomit? There are many causes of vomiting- some of which are dangerous, so if it is accompanied by fever, severe headache, if you are diabetic, if it could be due to drug overdose or if you have any chest pain- get in to the er now. If you think you have a good handle on the cause, then the most important thing is to take small sips of liquid often. Try some gatorade or "oral rehydration solution" (do an internet search for easy recipes). Get a shot glass and a timer- and take a shot every ten minutes. In general, vomiting twice is not nearly enough to become clinically dehydrated. But if you feel short of breath, or dizzy upon standing, you may well be dehydrated. There are some very effective anti-nausea medications, but not available over the counter.