If we want third party voices and third party ideas to get exposure then we need to open up the system. And there's one simple way of doing it - it's called preferential voting, AKA instant runoff election.

The short version of the cliff's notes is this:

Rather than presenting voters with a list of candidates, (a) (b) (c) (d) and allowing the voter to cast a single vote for their favorite candidate on the list - preferential voting allows voters to RANK the selections - I.E. select their first, second, third, etc. choice.

What this means is that rather than being unable to vote for a third party - because their vote would be "thrown away" and feeling trapped into voting for the "lesser evil", voters get to vote for their actual preferred candidate. If that candidate didn't get enough votes to actually win, then their second choice is automatically counted instead.

Here's a much longer explanation courtesy of Wikipedia: Preferential voting - Wikipedia, the free encyclopedia

After just a few election cycles, the two main parties would have to adapt and debate ideas from third party groups or risk being left by the side of the road.