In the UK there is an organisation called the National Council For Metal Detecting who publish a code of conduct :

NCMD Code of Conduct

1. Do not trespass. Obtain permission before venturing on to any land.
2. Respect the Country Code, leave gates and property as you find them and do not damage crops, frighten animals or disturb nesting birds.
3. Wherever the site, do not leave a mess or an unsafe surface for those who may follow. It is perfectly simple to extract a coin or other small object buried a few inches below the ground without digging a great hole. Use a suitable digging implement to cut a neat flap (do not remove the plug of earth entirely from the ground), extract the object, reinstate the grass, sand or soil carefully, and even you will have difficulty in locating the find spot again.
4. If you discover any live ammunition or any lethal object such as an unexploded bomb or mine, do not disturb it. Mark the site carefully and report the find to the local police and landowner.
5. Help keep Britain tidy. Safely dispose of refuse you come across.
6. Report all unusual historical finds to the landowner, and acquaint yourself with current NCMD policy relating to the Voluntary Reporting of Portable Antiquities in England and Wales and the mandatory reporting requirements in Scotland. See: http://www.treasuretrovescotland.co.uk/index.asp
7. Remember it is illegal for anyone to use a metal detector on a designated area (e.g. Scheduled Monuments (SM), Sites of Special Scientific Interest (SSSI), or Ministry of Defence property) without permission from the appropriate authority. It is also a condition of most agri-environment agreements that metal detecting access is subject to certain rules and regulations including mandatory finds recording. Details of these agreements and the access conditions they impose are detailed on the NCMD website.
8. Acquaint yourself with the terms and definitions used in the following documents: -(1) “Treasure” contained in the Treasure Act 1996 and its associated Code of Practice, making sure you understand your responsibilities. (2) Advice for Finders of Archaeological Objects including Treasure 2006. (3) The voluntary Code of Practise for Responsible Metal Detecting to which the NCMD is an endorsee. (4) Advice for finders in Scotland: see http://www.treasuretrovescotland.co.uk/html/finders.asp
9. Remember that when you are out with your metal detector you are an ambassador for our  hobby. Do nothing that might give it a bad name.
10. Never miss an opportunity to explain your hobby to anyone who asks about it. Appendix A to the NCMD Constitution Revised February 2000 Amended AGM June 2012

Most of this is just plain common sense however we can add to some of this from experience:

Be approachable.

Nothing worse than being a real miserable git if someone wants to come and have a chat , this is more likely to happen on the beach.

Don’t metal detect in the middle of the day on the beach

It is likely to be very busy and your just going to get in the way . Check your tide times and go at the end of the day when the tide is on its way out or very early in the morning.

Take all you find away

Despite all the news about lots of valuable finds and lots of jewellery on the beach these finds don’t happen all that often. I can guarantee that you will find a lot of rubbish , ring pulls , bottle tops, old nails . Take it all away , you are then also doing a service to the land as a kind of payback for being able to dig it.