In a democracy, how to ensure constructive criticism by the opposition, instead of mindless opposing for the sake of it?

we all need to realize that we have more in common than not, but I don't see that happening soon

If we could somehow make it to where elected office isn't an elevator to wealth we'd get a better class of people interested in serving. Unfortunately when maintaining power means increasing one's personal asset portfolio it attracts people who'll do anything to stay in office, including spending more time criticizing their political opponents than actually trying to make life better for their constituents. 

Be respectful, stick to the facts, and be prepared to agree to disagree because no matter what, people are entitled to their opinion which may not be rooted in facts. This is one reason why trump, who is chronically disconnected f/ truth/accuracy is so dangerous. His frequency created an alternate reality for people displeased w/ the way it is. 

For starters, you have accept at least some of the opposition's proposals as a reward for participating.  If instead you reject all their input and then complain when they don't participate next time, then you're just wasting everyone's time.