001. Join the right server.From my personal testing, I've found 8-12 player servers are the best. Anything over 12 is too crowded for me. The GES levels were mostly designed for a maximum of 16 players, anything over 16 is overkill.
002. Know when to run.If you have slappers, and you don't think your enemies are low on health, run! There's no point in running into some guy's bullets.
003. Know the item spawns.Once you're running away from certain death, you need to know where to run towards. Make a mental note of every weapon spawn you come across, you're going to need them later. Armor spawns are even more important, you should know where they are in every map.
004. Use body armor.Or you will die.
005. Aim for the head.In Goldeneye, hitting people in the face hurts a lot more than in the chest. A few well-placed shots will make you win the firefight.
006. Use your environment.If you're getting shot at, position yourself so that you're harder to shoot. Crouch behind boxes, move behind walls or other objects. If you're on cradle, you can jump off the side of the top and slide down the diagonal supports. This opens the map up a lot.
007. Use the right gun for your situation.Sometimes, a pistol is a better choice than a rifle. Keep in mind what makes each weapon good, and use that to your advantage.
And remember to have fun.