I'm a sinner. At one time or another, in the course of my own advocacy (on the death penalty and other issues), I have committed each of the five sins I am about to describe. In fact, so have most who work in advocacy, whether they are progressive, conservative, or located somewhere else along the political spectrum.
We live in a world that too often values conflict over solutions, and loud voices over wise ones. Avoiding the mistakes described below may not only make you more effective, but help make our public discourse more civil and productive.
1) Speaking mostly to those who agree with you
Over and over, I have seen the same pattern: an advocacy group gets some funding, and then uses the money to host a conference which gathers together large numbers of people who agree with the position of that advocacy group. It's rewarding, of course, and reaffirming, but what a waste of resources! If the point of advocacy is to change minds, it is almost always a mistake to direct our arguments to those who already agree with us. If you aren't talking to people who either disagree with you or haven't made up their mind, you aren't really doing advocacy work.
2) Confusing expression with advocacy
Many of those who work in advocacy have a right to be angry, and there is nothing wrong with expressing that anger. There is racism in this nation, and victims of that racism; there are a multitude of other injustices as well. It is not wrong to be angry about those injustices, or to express that anger.
Just be careful not to confuse the expression of that anger with advocacy which is directed toward those who disagree with you. Very rarely does having someone express anger at us convince us that we are wrong -- rather, it tends to drive us toward a further commitment to the very beliefs that have created that anger. That's how defensiveness works. This is especially harmful in the current political arena, where part of conservative identity consistently builds on the conviction that conservatives are under attack and victimized by their opponents and a biased media. If you lead with anger, you are merely pushing those who disagree with you deeper into their own position.
3) Not respecting the principles of the other side
If anger isn't advocacy, then what does count as effective advocacy? Well, first of all, good advocacy causes people to change their minds. That happens when we lead people to apply their most deeply held principles to the issue at hand. Too often, we challenge people's principles rather than the opinions they draw from those principles. Changing opinions is possible; changing someone's principles, especially from outside of their circle, is nearly impossible.
A better tack is to first understand those principles, then work with them. For example, gay activists find themselves opposed by conservative Christians who say they care about families and children. As someone who believes in gay marriage, I can accept this principle and work with it by, arguing that gay men and lesbians already have children and that those children are better off when their parents have the stability offered by marriage.
4) Failure to focus on outcomes
We shouldn't measure effective advocacy by the total number of words we utilize or the fervency of our speeches, but by the number of people we convince. Our political discourse is far too focused on "gotcha" sound bites, rather than those still, small moments when minds actually change.
There is a challenging selflessness to this, because in the end our advocacy is not about us, but about our audiences. It is they, in considering our arguments, who will determine our victories and our defeats.
5) Expecting capitulation
Finally, speaking of those still, small moments when minds actually change... we need to recognize that the effect of a good argument is not usually realized in the course of that argument. People don't change or establish their views based on the catchiest bumper stickers, or because someone manages to talk over their opponent on television. Rather, a good argument works on us over time, and often we do not realize that our minds are changing until we find ourselves using our (former) opponent's argument in a discussion with someone else. Don't begin a discussion hoping for a capitulation moment, when the person you are talking to begins to cry and submits to your will. Instead, humbly accept that if what you say is true and good, it will rest heavily on their minds as they process what you have said. Leaving a speech to an audience of those who oppose me, I am most heartened when my audience looks "troubled," because that is the true root of change.
If your deep need is to be affirmed by those on your side, then by all means speak only to them, while angrily disparaging your opponents. However, if you selflessly seek to change the world for the better, be prepared to understand and empathize with your opponent, step into their world, and only then ask them to walk out with you.