Don't try to be a hero on hard and suicidal difficulty; your team is better off with you when you are alive and killing infected, than when you are being eaten by Clots and not shooting your gun or swinging your katana.
Conserve your ammo; knife Clots when you can, and shoot the head off Gorefasts and Bloats. This will prevent them from using their special attack (Acid Puke for Bloat and sprint for Gorefast), giving you more time to focus on other things. They will also bleed to death by doing this. This can be done on most infected, but requires more than one shot on Husks, Stalkers, Scrakes and Fleshpounds (if you are not a sharpshooter).
Try to end your waves near the trader; this gives you more time to browse the list, heal teammates or share your money. This is especially important before the Patriarch wave, because then players have more time to set up pipe bombs.
Some general tips: Don't get closed in, keep moving, don't camp, keep alert, heal yourself whenever you can. (But give healing others priority, since then you get paid and it's more efficient to heal others then yourself)
If you're in a team of 6 players , make sure you always have atleast one sharp shooter and one medic