How Can I Improve My Mental Fitness?

Your mental health and fitness can be maintained in many ways.  Physical fitness is a good way to keep the mind clear.  A good diet is also vital, eating Omega Three and lots of fruit and vegetables is very good for mental fitness.

You could take a night school or distance learning class, or simply go to the library and take an interesting book.  Try an online mensa test or something like it.
Don't worry about feeling thick, it's the exercise you need.  You could join a website like MyBrainTrainer.com or something like that which encourages you to fully stimulate your brain on a daily basis.

Avoid anything that is too repetitive, it doesn't help mental fitness.

Try to get plenty of good sleep, drink water as regularly as possible, about eight to ten glasses per day will really help your concentration span and keep you sharp, which reminds me!