If you're ahead in your lane, you have to make your presence known, and sooner is better than later. It's nice to be 2/0 in mid lane or be up several kills as the support. However, if all you do from there is push the enemy team members to their turret and let them farm safely, you're not doing much to win the game as a whole.
What to do when you're ahead
As top lane, you generally do not roam as much even if you're fed. There are some exceptions, especially for playmaker characters like Shen (who rarely gets fed unless he uses his ult for ganks) or characters with the Teleport summoner spell. If you're fed and you're in top lane, you may want to consider stealing the enemy jungle camps (especially double golems if you're on blue side) or invading the enemy jungle to ward or set up invasions. If there are golden opportunities to make plays in mid lane, you can also do that. One thing I highly suggest for top lane is to use any excess power to buy wards, then use those wards to try to pick off the enemy jungler.
The ADC generally does not want to roam much, as farming for late game is extremely important. Once you've broken your tower, though, you'll want to look for opportunities to help a lane. Not all ADCs are well-suited for this, and aside from Jinx, there aren't many who make good roamers. If you're ahead, you'll want to look for any opportunity to assist in something, but as ADC you probably shouldn't be trying to make something happen yourself.
Sometimes, the support gets fed. I've had a friendly support Annie go 5/1 in lane, and it had a pretty profound effect. Annie was able to get our mid fed, she was able to help me shut down the enemy jungler, and she even went to top lane and set up kills there. If you're ever a support and you get very fed (possibly because your ADC is bad), try to get kills in other lanes so they can snowball. Having a large gold lead is nice, but if it's on the support, it's worst-case. The support must make plays if she is fed because that advantage won't last.
On the other hand, the mid lane has the most impact as a roamer. His expected lane is in the middle of the map, which gives him the ability to travel elsewhere much more quickly. If your mid lane is fed, she should not stay in lane.
I am not a big fan of Teleport because it reduces your ability to win your lane. However, if you win your lane despite having Teleport, you can easily get kills in other lanes simply by porting to wards. In low-level games, you can even teleport to minions; enemy players won't notice until they're dead.
Without Teleport, mid must walk between lanes, so it's a good idea to communicate (ask!) whether lanes are warded so that he doesn't waste his time. One thing I do a lot when I'm ganking is move through areas that are likely to be warded and view my opponents' reactions. I'm not always sure when the enemy is warding, so moving through those spots lets me know what kind of map awareness the enemy team has.
I realize that not all mid lane players are comfortable roaming, so I try to communicate via pings if I see anything interesting that I'd like people to go in on. For instance, if I see the enemy jungler coming to my side via a ward, I ping and try to let people know I want to counter his invasion. Once my mid laner has made a few plays in other lanes, she generally gets the idea that going to other lanes gets her more kills.
Wards are really useful for aggression. Whereas buying items that give direct power is nice, buying wards and putting them in places where you might be able to catch the enemy jungler can cause a very large swing in the game. If I'm support and I see the enemy jungler in top lane, I often try to ward the enemy jungle aggressively in response.
One thing that helps a lot is knowing timers. Generally buff timers occur about every five minutes or so, though those numbers fluctuate (due to the jungler's being elsewhere), especially during the mid game. Make sure that when you ward aggressively, the ward will line up with a likely time to take a buff. It doesn't do a lot of good to ward the enemy blue at 8 minutes in because your ward will be gone before blue respawns. Instead, warding at 10 or 11 minutes will catch the enemy blue timer unless the enemy jungler is very slow.
This doesn't mean don't invade or counter-jungle; you can still potentially catch the enemy at his wolf or wraith camp. You might also be able to spot a buff that hasn't been taken yet, especially if the enemy jungler is trying to gank frequently.
Roaming always carries a bit of risk, but the biggest risk is giving shutdown gold to an enemy lane that is already ahead. It is important that you not make things worse by showing up. I have had games that got worse because someone decided to give our blue buff to the enemy top lane. He made a failed tower dive with our blue buff and gave the enemy top lane a double kill (plus shutdown gold) and the blue buff. That game did not go well afterward.
If you're a high damage character (such as a mid lane mage) and you have a lot of gold, you really don't want to be the one deciding to dive turrets unless you're absolutely sure you can get away with it. Let someone else start the dive, or leave and go counter-jungle or roam elsewhere.
When you rotate to other lanes, make sure you pick the right timing and make the most of your advantage. Push out your lane and get your own lane on the defensive, then storm bottom lane and get your ADC some bonus gold. If you're 6/0 but your other lanes are struggling, you are probably going to lose. Make sure you do everything you can to distribute your power to others.
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