Get involved with local politics. E.g. the internal party selection process for a candidate for a seat in the UK parliament may only involve a few hundred people - if that seat is safe (i.e. practically guaranteed to that party) then those few hundred effectively choose the representative, and so have a large voice in determining policy.

(Ironically the one time I was one of the few hundred, our chosen candidate - who was excellent - got beaten by the Greens in an election upset)