What is the purpose of a jury? Robert Frost once said, "A jury consists of 12 persons chosen to decide who has the better lawyer", and all too often that is not a million miles from the truth.

The role of the jury is to listen diligently to the evidence, to consider the judicial summing up of the case, and to decide guilt or innocence entirely upon the facts and evidence that they have heard. That is the theory anyway. In practice, the jury decides within the first five minutes of the case who is guilty and who is not. They decide which witnesses they like (like, not believe) and which barrister they prefer (not like, they don't like either, but they may prefer one to the other). From there onwards, it is just a case of killing time until they can retire to keep everyone on tender hooks while they come to the decision they arrived at on day one, in the first five minutes.

Sadly, that is not too far from the truth. Studies seem to show that juries rarely engage fully with the case. An example is a recent case in which a juror was found to be in contempt for filing her nails during the presentation of evidence.

A more extreme example comes from the United States, where an obscenity trial was brought to a halt during a courtroom showing of the pornographic film Maximum Hardcore 7, when a member of the jury fell asleep. The judge declared a mistrial, that "justice requires the evidence to be reviewed and considered in its entirety".

In truth, both the prosecutor and the defence are desperate to get the jury on their side. The minute you lose the jury, you risk losing everything. For local authorities, it is doubly difficult because it is necessary to break down the prejudice that exists against the council, and to prevent the jury taking what may be their one and only chance to frustrate the council, as it has, in the past, frustrated them.

Getting a jury on your side requires a certain amount of stealth. Juries have a pretty boring job. They listen without comment and are sent out when there is a legal point, so they miss out on it. The judge invariable tells them to disregard the stuff that they would most like to have regard to, and then at the end of the trial, one of them gets to say five words, at most. Nothing wins them over like exhibits, which they can take back to the jury room and pore over at length. We are talking about simple exhibits here, not massive logs of computer data. Simple things like the bloodstained-knife, photographs or bags of "white powder" that can be passed around and handled. The more horrendous the photographs (subject, rather than quality) the better - and colour rather than black and white.

Something in a tagged plastic bag is always good, so that they have to fiddle with it to get a clear view, and anything sealed in a plastic box that they are specifically warned against opening is worth its weight in legal aid cheques.

The serious issue is that it is essential that the jury understand the case, and that they remember the important points when they retire to the jury room. The best way to do this is to give them articles of real evidence, in the form of photographs, plans or maps of the location, and real things that they can look at, weigh in their hands and scale against the rest of the scene.

Trainers constantly point to the value of training aides in getting a message across to students, and the same is true of a jury. The easier it is for them to understand an issue, or see it in their minds' eye, the more likely they are to remember it.

Conversely, the more difficult it is to understand a point, the more frustrated they become, and their frustration is taken out on the side making things difficult for them. It is an exceptional jury that will flog their way through hours of turgid evidence to find the truth behind it.

Many cases prosecuted by local authorities lend themselves to jury pandering. Counterfeiting and passing off cases generate real evidence, and health and safety, illegal meat, housing and even pollution cases usually make for good photographs.

Juries are not hardened to evidence in the way that local authority officers are. Showing them the evidence as it is can say more to them than a witness ever can, and more importantly can make them remember it. Michael Belmore said that juries are to be coaxed, teased and lured towards a verdict that they cannot resist, and that is about right.

Julie Barratt is director of CIEH Wales