Winning in one of UNESCO's Olympiads (Mathematics, Physics, Astronomy, Computer Science, Chemistry, Physics) is a guaranteed way to get into a very good university. These are international competitions, not nationals (not only in Poland as someone said).
anyway, I believe that theres no standard way to guarantee you an uncoditional offer. Helps if you have the asked grades beforehand (for example if you had just taken a year out) having some work experience may also help.