Unlimited Flares
Highlight 'Flare', enable it, but do not throw it down. When it is low, press Left or Right and highlight another object. Then, highlight 'Flare' again. It will have restarted and you will not lose another one.
Blood Test Kits
Save Blood Test Kits for showing your team that you are not infected to gain trust. Using them on a team member is useless because they get infected randomly, and usually break out within a minute of infection.
Blowtorch
Always use the Blowtorch while stepping backwards slowly, otherwise you will fry your feet.
Medi-packs
Always try to keep medics alive because medical packs can be few and far between. If you want your medic to heal a member of the team, tell your team to stay, then push your medic into the wounded team member. If he is hurt enough, the medic will heal him.
Mp5
When you begin the game, go back a couple of feet to find another MP5 machine gun.
Saving Ammunition
Try not to give all your team members the same weapon because your ammo will go quickly. A good combination is you with a Shotgun or Machine gun, your second man with the one you are not using, then a Pistol. The rest depends on your ammunition supply.