I know this thread has been inactive for a little while, but I think it's worth answering the question in case other people come looking for answers!
The truthful answer is yes and no. While it is seemingly not possible to successfully treat severe ADHD without the use of medications, it is absolutely possible to control your symptoms the vast majority of the time without medication like Adderall or Concerta. The clinical data is actually quite clear on this; manipulating variables such as diet, exercise and other "lifestyle factors" can have a profound and dramatic impact on the severity of ADD/ADHD symptoms.
The intake of sugar and foo additives should be avoided. Many food additives can exaccerbate symptoms of ADD/ADHD; much of the data on this is anecdotal but there is clinical evidence that certain preservatives an colorings can make both children and adults more distractable and irritable. So to prevent serious symptoms, kick the sweets and the pre-packaged, overly processed food.
Another thing to try might be nutraceuticals. These are naturally-occuring substances which can bring about positive changs in cognition. Many substances have been found to be effective for the treatment of ADHD symptoms. I'm not talking about anything particularly strange here. Things as simple and commonplace as Zinc, Iron, Omega 3, and Tyrosine (an amino acid) have been found to be effective for improving focus and reducing impulsive behaviors in people with mild ADHD. Then there are more unusual substances like Bacopa monnieri (a herb), Citicoline (a choline compound), and Theanine (an amino acid found in tea) which have also been shown to be successful in improving ADHD patient outcomes.
If you have been diagnosed with ADHD then it's worth following your doctors advice re medications. However, there's no harm in trying to reduce the need for the likes of Adderall or Concerta by reducing symptoms in a more "natural" way.