My top 5 tips for noobs, who were too much focused on leveling up the perks. So they already got level 5 or 6, playing on suicidal and HoE servers, but still don't know how to play. Couple of such "high leveled noobs" can totally ruin the good game.
So here are my tips:
1. Learn how to heal and to be healed
I recommend to rebind "Q" button from heal yourself to call for a medic (excluding solo gamers). When you healing yourself, you (unless you aren't medic):
a) waste all syringe to receive just 20hp;
b) unable to heal teammates for significant period of time;
c) become temporary vulnerable for further attacks (e.g. trying to heal up) near bunch of crawlers.
Typical situation:
you and your teammate are heavy wounded, you heal him and this retard heals himself too.
So heal yourself only if no teammates nearby or maybe when you are medic (due to very fast syringe recharge).
If you are wounded and there is a medic in your team:
a) call for medic (remember the key to bind)
b) don't panic - if you start jumping and strafing like idiot, it's practically impossible to do precise shot with a medic gun
c) stop moving (if possible) or move straitly to the medic (moving backwards let you to continue shooting enemies) to maximize healing shot precision.
If someone cries for medic, that means he needs healing, not just wants to see medic to say "hello"
So even if you are not a medic, but you are close, please heal teammate.
2. DON'T RAGE SCRAKES AND FPs with little amount of DAMAGE
DEMOMAN, please read title again. And again. And once more. Do you see underlined word? What does it says? DAMAGE.
Little damage, doesn't mean
small weapons and vice versa. Due to resistance to explosives and huge amount of health (on 6p game scrake has more hp than FP) scrake doesn't take much damage from your big explosive guns, but it will rage him. And there is no problem bigger for the team than pissed off scrake running out from the smoke cloud. So, demoman, if you see scrake, stop shooting and let snipers to do their job. In that time you can fully reload your M32 and prepare to clear out others zeds after scrake death to win time for your teammates to reload.
Other perks, this tip is for you too. Don't piss off scrakes. Let sniper do his job. On suicidal and HoE scrake must take 2 xbow headshots. First one always stuns him. So a good sniper gets enough time to reload, aim and do the second shot just after stun ends, but before scrake starts his rage-run. That's it - scrake is dead. Unless some retard takes scrake out of stun earlier by shooting it.
Same story with FP, but with one big exception - demoman, it's your guy. Blow him up, but do it smart. This strategy takes out FP just in couple of seconds even on HoE:
1. Sniper does xbow headshot.
2. FP starts raging. Every team member throws 1 grenade to him. Demoman, you can throw 2.
3. Demoman shoots with M32 in FP's upper body. FP is dead not even be able to do his rage-run.
Good sniper till this time can even make second headshot to FP. But it's not necessary. Just to be sure. When playing as demo I always make several shots to FP, just in case if teammates ran out grenades or sniper missed head shot.
3. Share money, but don't waste it
You supposed to share money with teammates, but not required to. It is your right to ask why someone needs money and where he's going to spend it. Maybe he wants to buy a weapon, and you can just drop it for him. For example, lvl6 supporter can get aa12 much cheaper than lvl4. So it's better for lvl4 to give money to lvl6 and ask for weapon than just flooding with message "I need money". Or medic to ask zerker for cheap katana, not just wasting 1000 on it. And demoman, I'm talking to you again
Don't waste pipes. It's ridiculous to waste ~200 pounds to blow up 5 clots or just one headless bloat...
4. On PAT wave do not weld the doors
I've tired to explain this. Just do not weld. If you want details, search the internet.
5. Do welding in shifts
Welding is most boring and unpaid task in Killing Floor. But in the same time it can be most important warrant for team's survival. If you need to do welding during entire game, it's a good idea to plan shifts. For example, change each other every 25 zeds killed. Or if someone runs out of ammo or get seriously injured, he immediately goes welding without waiting his turn. Then it is fair, easy to control and not so boring.