There are many tasks that successful support players need to do. Firstly, it is their main job to secure map control for the team by warding as much as possible, but also warning the team about where the enemy team have vision. Next, the support need to protect his carries in teamfights, and make sure they stay alive until the fight finishes. Also, what you can do as a support player to help your team get advantage, is roam.

When to roam

There is no certain rule when to roam, but there are some tips that could be used as help in making the decision whether to roam or not:

- Communicate with your AD Carry: Before you decide to roam, u certainly have to make a deal with your carry that he should not push, because he could get caught in 1v2 scenario. Be sure that your carry approves your decision to leave him alone in his lane, because he may be feeling unsafe to stay 1v2. So making a deal with your carry is vital before leaving your lane. 

(preferable wards if you are blue team, the ward in the second picture prevents lane gank)

- Ward the entrances to your lane: After you are sure that your AD Carry accept your decision to roam, it is also preferable to ward the entrances to your lane. Even if you made a deal with your carry to stay defensive, that does not mean that he cannot get towerdived by three people (if the enemy jungler shows). For that matter, it is preferable to ward the entrances to your lane, so your AD Carry have information about possible dives, and give him time to react. 

- Get information about the enemy laner: If you are going to roam and help your mid laner, it is preferable to ask him whether the enemy mid laner has his Summoner Spells available or not. If he has both of his Summoner Spells available, do not hesitate to roam and gank that lane. Forcing him to use both of his Summoner Spells is almost equally worth as a kill, because it opens a window afterwards for you to roam again, but also creates easier time for you jungler, when he decides to gank that lane. If the enemy mid laner does not have his Summoner Spells, only goes in your favor, because it increases the chances of successful gank.

(preferable wards if you are red team, the ward in second picture prevents lane gank)

When not to roam

- When your AD Carry wants you to stay in lane: A lot of the AD Carry players are not used to stay in 1v2 bot lane. So if they do not agree with your decision to leave the lane, respect his call and stay with him. Disrespecting his calls, and low communication between the two of you, will destroy the so called botlane synergy, and will pretty much guarantee you having a bad game.

- If the enemy team has high poke/engage botlane: If you let your AD Carry alone against the combination of poke/engage supports (ex. Annie, Thresh, Leona) and poke AD Carries (ex. Ezreal, Caitlyn), he certainly will have bad time on that lane. He could get zoned out of the minion range which can lead him to fall behind the enemy AD Carry. A lot of players would say that we see that kind of supports almost every game, so does that mean that it is impossible to roam in current meta?

Definitely not. The best time to roam if you are facing poke/engage kind of botlane, is after you force one of them to go back to base (preferably their AD Carry). Try to use that window when your AD Carry is left 1v1 against the enemy Support and can easily farm, and quickly rotate and try to help your mid laner. On the other hand, if you force the enemy Support to recall, you can decide whether to stick in your lane and try to zone out the enemy AD Carry, or rotate to mid and let your AD Carry in a 1v1 duel. This depends on the respective champions that each of the AD Carries are playing.

Art credit: http://geekgilmu.deviantart.com/art/LoL-Nautilus-338223575

Playing against a support that is roaming a lot

Lately we see that a lot of Support players tend to roam a lot, whenever they have a chance for it. Playing against those kind of players is not really easy, but good coordinaion and constant calls of their position should help you a lot.

First thing to do, and most important, ping (or write ss in chat) every time you see the enemy support leaving the lane. With that you give information to all your laners that it is possible that the enemy support will show at any other lane.

If you spot the enemy support going to rotate to mid lane, there are two things that you can do:

- Stay in lane and zone out the enemy AD Carry: this is the option your AD Carry will love a lot, as he will have great time farming without any pressure. What is more important, he can get CS advantage over the enemy AD Carry, or in the best case, get a kill. However, you need to spot the enemy Support in the mid lane before diving the enemy AD Carry, because if he is close to bot lane and comes back in time, 2v2 under their turret can only finish disastrous for you.

Art credit: http://ewalabak.deviantart.com/art/League-of-Legends-Leona-393497834

- Follow the enemy support: this is the option that your team would apreciate more, but to do it correctly you need proper vision on the map. You definitely do not want to get caught out roaming after the enemy support in dark areas, because that could cost your life. If you do not have proper vision set on the map, it is better and wiser to just warn you mid laner to be careful, go back to the bot lane and pressure the enemy AD Carry.

However, at the end of the day, it is the vision control that should lead your next call. If you spot their jungler in top side of the jungle and their support is close to mid lane, to not hesitate to follow him, as they will end in 2v3 situation. If the other situation happen, if their jungler is closer to mid lane than your, just ping your mid laner to hug his turret and go back to bot lane.

Roaming is pretty much a vision and mind game. The team that has better vision and do better rotations around the map using the vision they have, will certainly have more chances to get the lead.

Best support champions that are good at roaming

The best champions for roaming are those who have spells as stun, knock up, bind, or lock a target down with chain of spells. Combined with high burst assasins (ex. Diana, Zed, Talon), can almost guarantee a kill or at least forcing the enemy mid laner using one, or both of his Summoner Spells. So, here are the best champions that can be used to roam:

- Leona

Leona is a champion that brings a lot of CC to the table before level 6, but also once she reach level 6, she has a combination of three stuns, which is why she can do really successful in roaming.

Art credit: http://justduet.deviantart.com/art/Thresh-369995463

- Thresh

Thresh is a champion that provide a little more than the other champs, because even if your mid laner is out of his spell range, you can bring him close to the enemy target with your W.

- Nautilus

Nautilus has made his way through to the top picks lately, mainly because of his skill set. Because of the fact that the other champions need to land their skillshots, Nautilus has one advantage, his R, a spell which is impossible to miss (except of course if you do not have range to cast it).

- Alistar

Alistar also rely on hitting his W+Q combo, but when landed successfully, opens a window where the enemy target can be burst down really fast.

Anyway, these are not the only champions that are good when roaming. Of course even a successful Morgana's Q, or a hook from Blitzcrank, will get the job done. However, it takes a lot of practice to learn each champion kit (ex. Blitzcrank's Q lands faster than Thresh's Q) so do not get disappointed if your roam is not succesful the first time.

Conclusion

To sum it all up, roaming is one way for the support to help his team get advantage. For good roam it is not enough only the support to do good job, but also the whole team shall contribute as well. It requires a lot of practice to read and use the enemy positioning and vision coverage, so if you do not success at roaming at first, do not worry, your help to the other lanes will be more and more successful with the time you will spend practicing.