"A Cry against Injustice" is the title of this Psalm. When I read "God knock their teeth out" I was taken back a bit. It seems like such harsh and graphic language for us sensible Americans. But I think the point is that as a lion's fangs are it's strength to destroy it's prey, so David is asking God to take away the ability of those who are practicing injustice to continue.
We should pray a similar prayer when we see injustice. The poor, the downtrodden, the weak, the homeless, those afflicted with addictions (even though it is usually their own bad choices that got them there) - the list could go on. All of these - even if their troubles are from their own choosing - deserve justice in their lives. We should pray for them and work for them that those in power do not take advantage of them and that they are judged fairly.
How do we do this? I guess it's a lot easier to say we need to take a stand for justice than to actually do something about it. My first thoughts are that we need to elect politicians who will make sure our court system is working justly and honestly. We need to elect a president who won't appoint judges who legislate from the bench. We should do what we can to get someone to replace the judges on the 9th Circuit Court of Appeals (the court doing the most damage to Christians values).