They will try to intimidate you, and convince you that you have violated a law and will be fined hundreds of dollars.
They will ask for your driver's license and registration, and eventually suggest that you can pay them directly so you can get on your way quickly.
Here is the problem, paying a police officer directly is against the law, and it amounts to soliciting bribery. If you do so, both you and the officer are breaking the law.
If I believe I have done nothing wrong, I simply refuse to pay. I do so without getting angry, rude or confrontational. I hand them this statement in Spanish.

AVISO A USTED OFICIAL DE POLICIA
Entiendo que Usted ha pedido que me detenga debido a que posiblemente he cometido una violación de tránsito. Obedeceré sus instrucciones de acuerdo a la ley, le entregaré mi licencia para conducir y el registro de mi vehículo. Sin admitir culpa en esta situación, voluntariamente lo seguiré hacia la Comandancia de Policía en donde Usted o alguien podrá informarme acerca de los procedimientos legales a que haya lugar y si me es requerido, pagaré la multa que corresponda. Entiendo que en este caso, se me extenderá un recibo por el concepto de la multa.
Translation:

NOTICE TO MEXICAN POLICE OFFICERS
I understand that you have stopped me for allegedly committing a traffic violation. I will obey your legal instructions by providing you with my Drivers License and vehicle registration. Without admitting guilt in this situation, I will voluntarily follow you to your local police station, where you can properly inform me of the legal procedures, and if asked, I will pay the required fine. I understand that you will issue me a receipt for these actions.

You can print multiple copies of this statement using this link. -- Mex. Police Statement
Print two sided and cut up into 4 pieces , that way the English is on the opposite side of the Spanish Statement.
There is a second statement in this document which I'm not sure that I would use.

I carry a laminated copy of my driver's license. I politely refuse to submit my real license.

Here is a form some people try to get the police to fill out. -- Mexican Police Form

Here is another similar form I found. -- Mexican Mordida Form
Most of the time, they will just hand this back and say go.

Read Please give me my ticket. “Dame mi multa, por favor.“

Read Kevin's Blog on the same subject.

Read this Lonely Planet Article - - Is It Safe In Mexico?