Karthus: As soon as you die (if you do) you should turn on defile and start spamming lay wastes to try and do as much damage as you can before you go. That much is obvious (to some) but what isn't obvious is that if you start to cast your ult even with only 1 second left on death defy it should still go off.
Clairvoyance: Not many people pick this spell and because of it nobody notices when they're being spied upon. There's a huge eye floating above the part of the map they revealed. If somebody on the other team has picked this spell look for it. It's often a tell tale sign that the other team is looking for you to gank, so if you see one pop above your head while you're in a bush get out of there fast. This summoner spell is amazing for heros like gangplank, karthus, ashe, and other people with map wide ults.
SkillShots: Skill shots are often regarded as an annoyance, as you can miss and the effect is null. But they're also very handy for searching for stealthed heroes. A good example for this is Dr.Mundo, he has a very cheap and good cleaver attack that he can use every 6 seconds. if you think you are being stalked throw a cleaver in the path of your retreat (a place where twitch likes to hide) if you hear a squelching sound run away from that sound. Or behind you (a place where eve likes to hide). If you don't have wards or a potion any good stealth player will know right away and will have no fear in going into either of these two places. What they don't expect is Mr.Choppy to come sailing their way (they'll call you a hacker, just respond with "don't be so predictable.) Best case senario act like you have a ward up and they'll be less likely to gank in that location.
This is also amazing for checking bushes. Toss a cleaver along the disance of the bush and not only have you checked it for enemys but you've also slowed any would-be gankers so they cant follow you. If the people in the bush are particually dumb they'll just stand there and hope you didn't notice them, allowing you to toss another cleaver in a few seconds later. I use this EVERY time I go to top or bottom at the start of the round. Most players will wait untill you have entered the bush and can see them to attack. So you can get pretty close and most likely if they're in the bush they'll be hit. If not feel free to set up your own gank.
If you're Morg then when you hear your skill shot hit, cast a defile ground at that location, and all of a sudden the ganker becomes the ganked.