How do I stay on the neutral path? do I need to have a balance in crucial choices?
This is a simple question with an overly complicated answer.
Garden variety Neutral Affinity requires keeping the total number of DIK and CHICK Major Decisions (such as whether to join Quinn's restaurant or to fight Tommy) relatively balanced. If you have made 2 or more CHICK decisions than you have DIK you will gain CHICK Affinity (and vice versa).
But that's only a temporary Affinity; it can change based on what you choose in later Major Decisions. At the end of Episode 8, we gain
permanent Affinity and the rules change slightly. Permanent Neutral Affinity requires
not gaining permanent DIK or CHICK Affinities, which means you need to have at least one 'box' left on each side of the bar after the 15th Major Decision (whether to cheat on the midterms). That effectively requires making a minimum of 5 Major Decisions for each side.
So to recap: for Seasons 1 & 2, Neutral Affinity requires making an almost equal number of DIK and CHICK Major Decisions; for Season 3, Neutral Affinity will require having made a minimum of 5 CHICK Major Decisions and 5 DIK Major Decisions.