TL:DR: If you are on a jury, don’t convict the defendant unless what they did was morally wrong.
Ideally, laws would conform to societal norms. If someone did something that the vast majority of people thought was wrong and worthy of punishment, they would be punished. If not, they would not. The American criminal code is really long and complicated. People might not even know about the laws they are accused of breaking, and everyone commits minor crimes. About 34% of the prison population is from victimless crime. Privatized prisons create a vicious cycle where they pay politicians to imprison more people, and use the money they are paid to lock up criminals to buy off more politicians.
There are many ways to try to alleviate this problem, such as political activity and whatnot, but there is something simpler that everyone can do which would solve the problem without any political reform: jury nullification. Jury nullification occurs when a jury finds a defendant not guilty, even when there is conclusive evidence, because the law itself is unjust. The point of using a lay jury is to bring an outsider’s moral perspective into the legal system.
If the trial were simply a fact finding excercise, there would be no need for either juries or even lawyers. Judges could do everything themselves. Juries act as a check on the abuse of power, both by corrupt judges and by corrupt legislatures. Juries do not have to disclose why they came to the verdict they did. If you are in a trial, do not flaunt that you intend to nullify, because you might be removed by the prosecution’s lawyer. However, do not worry that you will be punished if you do nullify an unjust law.
I decided I should elaborate on my societal norms comment at the beginning, after some comments on Google +. When I say “societal norms”, I mean a situation where over 95% of the population agree with a crime. 99.99% of Americans agree that murder, rape, theft, assault, and fraud are wrong. If you take a random sample of 12 Americans, you will very likely get all 12 in agreement on the wrongness of murder. Knowledge of jury nullification will have roughly no impact on conviction rates for murder. Only about half of Americans support criminalizing marijuana possession for adults, and even fewer support criminializing medical marijuana. Even if only 25% of Americans know about jury nullification, and 50% of those nullify charges for marijuana, the chances of conviction drops to around 20.14%. If half of the population knows about jury nullification, and half disagree with the crime, the conviction rate will be only 3.17%. I doubt prosecutors will be willing to go through the trouble of 5+ trials just to convict someone of such a crime.