What can I do to avoid an infraction for possession of marijuana?

First of all, possession of MJ is not an infraction, it is a criminal misdemeanor. That's why it is a good idea to get a lawyer who may be able to have it reduced through negotiating with the Judge/prosecutor.

If you plead guilty to this crime it is equal to a speeding ticket and the fine is $100. Nobody will know that you plead guilty except a police officer or prosecutor if you get in trouble again. With that being said, there are time that you can negotiate a Conditional Plea. This is when you enter a plea of guilty however sentencing is continued for a period of time. Sometimes it is six months other times it is one year, it is up to the judge or the officer. It is unlikely that a district attorney will be present because they usually do not appear on infractions. If you accept a conditional plea you will have to go back to court after the time has gone by. As long as you have not been charged with a new crime you will be allowed to withdraw your plea and the case will be dismissed.

Take it to trial. Don't settle. Force the case to go to trial. The officer may not show up, he may not prove the case, but even if he does and you lose they can't punish you more harshly.

An infraction is a good deal. Similar to a traffic ticket, you just pay a small fine and avoid jail / probation. After one year, you can simply expunge the conviction. You may be eligible for diversion, whereby you would be ordered to pay fines, take classes, submit to drug tests and stay out of trouble to get the case dismissed, but that's probably more time / money / effort than it's worth since possession penalties were reduced.