1) Don't vote for the looneys. That includes all Conservative, Green and New Zealand First party candidates.
2) Vote for the ACT candidate if you have one in your electorate. If you don't, vote for the National candidate. Yes, I know, voting for that bunch of unprincipled wimps in the National Party sticks in my craw as well, but as bad as they are, the others are worse.
3) Give your party vote to ACT.
Simple really. And remember the old Democratic Party slogan, vote early and often!