I would suggest using premium gasoline. Brake fluid should be changed every two years or at 30k miles. Only clean, fresh fluid should be used that has not been exposed to the atmosphere. Keeping the brake fluid reservoir closed will help prevent moisture contamination. There are also test strips you can purchase at any auto parts store that will allow you to test your brake fluid for moisture contamination.
Again, I would use premium fuel and keep the tank above 1/2 tank full. I would change your oil once per year and check the brake fluid with the test strips I mentioned and see if it is moisture laden. If the brake fluid is saturated with moisture, the system should be flushed and replenished with clean brake fluid of the type recommneded by the manufacturer.
i used premium gas the first 6 months, then after that, i've been using 87 octane (the last 3-1/2 years)in the winter i get 40mpgs and 50mpgs in the summer. i noticed no difference in performance or mileage. next saturday is my 4 year anniversary of smart ownership :)