Don't rage quit

If you get beat, don't just quit the game - especially if you're in an alliance. It causes the whole game to end.

Instead, ask the game host to kick you, or just stay around to watch the rest of the game.

Note: If you are the game host, you can't exit without causing the game to end for everyone else. So stick around until they're done!

If you're in a fixed teams game, ask an ally for a truck or power to see if you can get back in the game?

Don't force a winner to play "hide and seek"

If someone has destroyed all your forces, except for a couple of units, don't make them spend half an hour hunting round the map to find your remaining units.

Say "gg" to let them know you're beat and then the game can end. Alternatively send your units to their base so they can finish them off.

Kicking players

This can be a contentious issue, but at the end of the day a laggy player will make the game horrible for everyone else.

Rommel had this to say on the subject:

The main thing is that you should monitor pings properly before the game starts, if someone has a ping that is spiking over 500 and averaging over 300 (some would say 200) then I normally ask the person politely to leave (explaining why). Normally people leave before needing to be asked as they can see their ping is high. Always be careful here though, as joining players will spike all pings - you should wait a moment after a new player has joined before making a decision, also pings bounce around a bit so it might only be a momentary [lag]. One sure sign is if you see the asterix where their ping should be, if this is flicking on and off a lot with a high ping they should be asked to leave.

For more discussion on this subject, see: Do you like the kick?