Minecraft wiki entry for Multiplayer_survival, associated text: Try to log on when almost no players are online for the first time to run from spawn, like in the middle of the night, there may be players camping near spawn killing all new players. You do not need to worry about this if you can be teleported to a random location upon spawn.
Have your best gear at all times to fight off enemies trying to kill you
Try not to have your base near spawn or heavily populated areas. If possible, hide your base and remember the coordinates.
If your friends are joining with you, make an alliance. It is risky to do this with strangers, but it is a good way for new friendships to be made (and broken if they betray you!)
It may still be possible to trade, but only on a few servers.
Do not participate in wars unless you are being threatened or your friends are involved. Unnecessary wars can needlessly damage or destroy your gear or damage your base, possibly with no make-up for it at the end even if you are victorious.
Be ready for the day someone finds your base and decides to destroy it and loot the contents. Unless the server has anti-griefing rules and can restore to a backup, the damage will be permanent.
It is also advisable that if you see another player, you should mostly keep sneaking and standing repeatedly. This is considered a sign of peace, and there is a good chance that the other player will not attack you, unless you have harmed him in any way, or griefed his base. If that player has griefed your base or harmed you, you can attack him, but also beware of the consequences it may cause.
An efficient villager trading hall with weaponsmith, toolsmith, and armorer villagers selling diamond gear can be a great addition, along with a villager to obtain lots of emeralds from. Ideally, you would also have librarians selling high-level enchantments, but this is hard to set up.
If you raid an enemy base, always look for traps. These include random valuable blocks to trigger a trap, a pressure plate (even in front of an iron door), a storage container that is linekd up to redstone, a tripwire, or a switch. If their entire floor is carpet, break a piece (not the one you are standing on). Carpets usually hide pitfalls or lava underneath, and can quickly fall if their supporting blocks are removed. You should also watch out if they built their floor from gravity-affected blocks, because they could be one update away from sending the player into a pit.