Try not to obstruct other players in the game. Intentionally blocking access to industries, large areas of the map or areas that another player is building in with your own track, roads, or with the buy land tool is generally frowned upon. Buying exclusive rights in a town in which you have no stake is also frowned upon.
Don't cheat to advance yourself within the game. Joining an unused company or starting another company and then transferring money to your own company is a no-no. If you discover an exploit which gives you untold wealth, report it to the OpenTTD developers so the vulnerability can be closed.
Avoid using obscenities. OpenTTD is a game which is enjoyed by people of all ages and obscenities are not normally necessary.
Don't get revenge. If other people buy all land around or cut you off with rails, do not get 'revenge' by blocking their stations or any of the above. This makes you one of them.
Try not to build long diagonal tracks over long distances because they cannot be bridged and avoid signals close together so that bridges can be built over the tracks.
Sabotage of other players (deliberate, excessive land purchase to block rights of way, stations, etc) is not acceptable behavior. Players doing this can be barred from the server. The only allowable forms of sabotage are through the local authority options.
Do not password your company if you don't intend to play anymore then un-password it to allow others to get on the server. To remove a password, enter a blank password instead.
By not following these guidelines you may, at the least, annoy other players. Players who continue to play unfairly may find themselves warned, kicked or even banned from a server by a server operator.