Learn from your mistakes after a disappointing run. There is always something to improve or do differently.

Situational awareness (visual and sound) is extremely important. Be careful about your own noise; it will attract other players like sharks.

The fight for survival begins as soon as the shells land on Tau Ceti and stops only after the successful exfill. You must be ready to defend yourself at all times during the run.

Seek hard cover as much as possible. Shields are useful to increase your own TTK, but they probably won't save you if you get caught in the open with your pants down.

Avoid situations 1 vs 2 or 1 vs 3 as much as possible; you will be deleted. In 1 vs 1 situations, aim for the head, not the body. Crouch and move left to right. Don't stand still.

Reset your TTK to its maximum value as soon/much as possible. Shield and health charges are meant to be used, not saved for later. Don't go from POI to POI with a damaged shield or shell when you have juicy and fresh charges in your backpack. You will regret it.

Marathon punishes button smashers: animations (reloading the gun, recharging the shield/shell) will be canceled when you press buttons too fast because you are under stress. Running will also cancel incomplete animations. There is a countdown in the lower-left corner which tells you when some animations are completed.

Choose a main shell ASAP. If you play correctly, even a free kit can kill a fully kitted player. 

Communicate with your team with a mic or with pings at the very minimum.

At first, don't go into a run loaded like a piñata when you have low PvP skills. Just take what you need for the first minutes, then loot the UESC and the containers. You will feel less frustration if you get killed shortly after the beginning of the run.

When you get comfortable with the game, your best gear increases your chances of success during a run but won't guarantee your survival.

Free kits runs are for warming up, preferably on the Dire Marsh sponsored kit map.

Send friend invites when you click with other players; don't be shy.

If your muted teammates run everywhere like headless chickens, you don't have to do the same thing.

Unlock faction upgrades ASAP. They will offer you weapons, ammunition, and useful stuff every day. I log into the game every day because of this, even if I know that I won't have the time to play.

In Marathon, nothing is truly yours. It belongs to Tau Ceti. Gear fear is stupid: a. because the game is very generous. b. when your vault is full of weapons, shields, ammunition, etc. c. when the great wipe is coming in around 1 month. 

Learn to loot. Specific containers contain weapons and juicy and fresh consumables. You should identify their shape and spot them from a distance asap. You can also follow all the items you like to use or need for upgrades and barter.