I swear, every time. I don't know if it's something about the initiative structure or what, but sometimes it feels like the party's plan always falls apart the moment one little thing isn't exactly right. If the plan is PC 1 - throw a flask of oil, PC 2 - cast fireball, if PC 3 goes before 2, they're inevitably going to just charge into melee.
For player tactics, I'd start with a simple phalanx. Rather everyone going wherever, wait for the enemies to come to you. I've seen so many instances where a PC has gone to making death saves because the cleric wasn't close enough to heal, or where the wizard was left open to melee because the tank took the battle to the enemies. Just use the ready action and stay in place. You might not get extra attack, but sometimes you have to sacrifice offense for defense.