Just some idea I've come up with...
Suppose you can exercise your mental functions, and by using them they get stronger. I don't know if this is true, but at any rate, here are some exercises!
Se: imagine yourself as an alien coming for the first time on earth. Describe everything you observe in great detail. The catch: don't start to compare with your home planet, because then you're using Ne... I'm so going to try this.
Fi: try to describe, bring under words, how you feel right now. Stick with the description, don't search for reasons. Decide if you want to keep this feeling or not. There's nothing wrong with a little introspection, too. If you don't feel at ease introspecting, you shouldn't do it. Let Fi decide.
Te: define some goals to reach. Write them down if that helps. Try, consciously, to find the best way to reach those goals. If this helps, write down the steps you'll take.
Ni: whenever I think Ni, I think "heuristics". Get some problems to solve - with "problems" defined as questions you don't know the answer directly. These can be math problems, but also trying to cook a meal with some given ingredients with which you don't know a recipe right away.
Si: try to do something in a traditional way, just this once.
Fe: before you post something on this forum, try to imagine what effect it would have on others. Would it help them or piss them of? How will they interpret your words? If you answer a question, will your answer be helpful?
Ti: before you post something on this forum, check consistency.
Ne: why would you want to make your Ne even stronger?!
Well, okay... take the Se exercise but do imagine what's different at your home planet.
Any other exercises?
If you try them, what are the results?