Never offer or give money to an officer. Extortion is illegal and agreeing to pay can make the situation worse, you only encourage them to continue taking advantage of others.

If you are charged a fine, request your official receipt from the municipality.

If possible, record the interaction with your mobile phone. Do it discreetly if possible. 

Doing it live or sending the evidence immediately is useful to avoid being forced to delete it and to have tools to raise your voice

If there are people around, try to have witnesses present who can corroborate your version of events or who can help you. 

Try not to be alone or expose yourself in risky places or times

Try not to stay alone in isolated places. Move the interaction to a public, bright place.

Most events occur while alone, in dark, lonely places and at night.

If they ask to search your belongings, politely ask if they have a court order to support their request.

They can't go through your things or search you. Keep your distance, don't let them touch you.

Avoid possessing drugs and alcoholic beverages on the beach, street or public spaces. 

Seek to be in private establishments and accompanied.