I didn't mean to make it so long, it just happened.
As Mundo - Sadism is a great spell, but if your low on hp, use it. The short CD will be back up in no time and gives you great lane control.
As Fiddlesticks - Hide in the bushes and make sure you use your ult where you hit the most people.
Never use drain life if you ult because you can't move and the range is longer then your ult's range, meaning that the person you were focusing is gone.
As Signed -
When being chased, always have poison trail up to make them take damage. If your attacking someone, it's always better to be in front of them so that poison trail will hurt them, and signed's auto attack is pretty bad even with your ult up.
Get one level in mega adhesive so that you can set up a slow/fling combo.
When you fling, they will almost always go a different way if a path is easier, so either make this path inpassable (mega adhesive) or follow them.
Getting FoN even when they don't have alot/any casters can be useful if you have swifties so that you chase down and fling any enemy heroes for your friends to kill. Useful for those tower huggers.
Mana is your best friend as Signed. I see too many people start out with Health Crystal instead of Mana Crystal.
Using poison trail when a new enemy minion wave is coming can be used for farming easily. Get ready for the wave to come, then put on poison trail and run away from them. You'll take no damage, and you'll likely get a few minion kills.
Playing as Twisted Fate - Red cards can be used for harrasment against any enemy champs by hitting an enemy minion close to the champ.
Destiny can be used if all champs are MIA to see what theyre doing.
Playing against Karthus - Get magic resistance. I don't understand why, but whenever there's a karth, nobody finds the need to buy magic resistance. If your caster buy abysall specter, if your dps, buy hexdrinker, and if your tank, buy any magic resistance items.
When you kill Karth, don't stay near him, he will constantly be casting lay waste and have his defile up.
General Tips -
Positioning is very important. If your caught out of position it can lead to ganks, deaths, and aces.
Wards are great, as they prepare you for enemy ganks, or to see if their enemy is jungling.
Teams should have a normal composition of a Tank, Physical Dps, Mage DPS, Support, and a jungler.
Map awareness is important. Always pay attention for MIAs and check the mini-map often to see if anybody is MIA. If you see someone MIA, talk to your team to see if he's missing, or is healing, and call out MIAs, even if their just healing.
Be supportive! A simple rule, and sometimes I don't follow it as well. Yelling at a teammate will almost never make them play any better. If their doing bad, say so in a nice way and try to tell them how to fix what they're doing. If their not accepting your help, then it might be better to give up. If they do accept it, then give them the advice to be a better player, and hopefully they'll take it to heart in this game, or maybe the next game.
Know your self and your enemies. Every champ has a strength and weakness. Change your build to what is happening. Lets say one game as malphite, they had tons of physical dps, so you built thornmail and armor. But the next game they had more caster, so you built towards a banshee veils and magic resistance. Know your counters and work to block them. As a mage, buy magic pent, while as a melee dps, buy armor pent. If you see someone with thornmail, tell your mages to focus them, and if you have no mages, then that's your fault for not picking a good team.
Item Tips - Sunfire capes can make nuetral creeps attack you, so try to ignore jungles if your at low ho.
Quicksilver Sash is an unrecongized item, useful for people getting focused by stuns, its essentially an extra cleanse thats very cheap, and gives pretty good magic resistance.