First of all the easiest thing you can do is just to keep in mind where all your teammates are and what they are doing, for you not to get surprised and jump scared when they pop up infront of you while you are shooting or changing directions.
One of my favorite ways to not deal dammage to your teammates is to add mods to the charecters, most of the time the reason for friendly fire is because you mistake them for zombies, expecially in Left4Dead games the charecters are very similar to the zombies in the game, for example i always mistake Coach for the Boomer. To combat this i like to add the most colourful charecter mods to the game such as anime charecters or other game charecters with brightly coloured outfits. You can easily add mods from the addons part under the game in your library.
After you have added the mods of your choice you can see that you barely ever shoot your friends, sure you'll always shoot them for fun time to time but other than that you never shoot them on accsident.

Thanks for reading my guide if you enjoyed it i would appreciate it a lot if you could give it a thumbs up and give me rewards (I need ponitsz plox) :3