Let’s start with the biggest no-no. Trying to do Baron or Elder without vision of the enemy team. Forcing a Baron can easily turn a game sideways. Picture this. Baron takes everyone on your team’s health. The enemy team (even though they are down by 15 kills) waltzes in and aces your team, while taking Baron in the process. This is something that can easily be avoided.
Prepare the objective. Do NOT try to rush Baron or Elder just because you do not see the enemy team and think you could do it. If you are worried about the enemy team running to Baron to stop you in time, it probably is not a good idea to do so in the first place. The short amount of time it takes to prepare the objective may help decide if doing Baron or Elder is a good idea. When I say prepare the objective, I mean take scuttle, clear out enemy wards (use control wards, not just sweepers!), place wards in the enemy team’s jungle, and take the blast cone away if on the enemy team’s side. Taking away blast cone takes away the ability to Flash out of the objective pit in a steal attempt, as it would have to be used to enter the pit on most champions. Preparing the objective should be a team effort! If the enemy team shows up in this time, leave the objective alone and take the vision control you gained of the objective. Here is an example of important warding locations (note the color of the wards, you obviously want to ward the enemy side).
If they have not yet come, start doing the objective. The vision gained should provide ample time to get off of the objective without having someone be picked off. Essentially at this point, you have left the enemy team with a decision to make. Do they let you take the objective for free or do they face check the area?
The latter option could force the enemy time to fight blind. Do not let your vision go to waste, however, by trying to just burst down Baron or Elder. Get off the objective and zone the area. Support and Top can keep a couple enemy champions away from the area for your damage to finish off Baron or pick-off a isolated enemy trying to be a hero. If the whole enemy team shows up and enters river, everyone needs to get off Baron or Elder and fight. The only exception is if Baron or Elder is low, in which case the Jungler should remain on the objective to Smite. Do not always remain on the objective. It can always be taken after the team fight, if one is going to occur. A half-done Baron taking your health, while your team tries to 4v5, is a recipe for disaster.
Communication and vision. These two aspects come in handy in all game situations, whether winning or losing. Get deep vision into enemy jungle, ward objectives beforehand as discussed, and, if split pushing, ward to be able to leave safely so you do not get caught out. Always have a control ward on the map, no matter what your role. Ping and use chat. Let your team know what you are going to be doing, so they can play around it. Communicate the opposite! Your jungler trying to rush an objective for no reason? Ping careful and say bad idea! Do not spam, however. Keep messages short and sweet, while avoiding trying to tilt anyone. Give instructions to help your team out. Ask your Support to use Sweeper to clear wards or have someone take the blast cone out. Going to split push as a Top? Let your team know and say you have Teleport for a fight.
Another way to close out games is to push other lanes when an enemy inhibitor is taken. A common mistake in lower elo is to continue to push a lane which already has an inhibitor taken. Let the Super Minions act as a sixth champion and push the lane themselves. This will allow your team to potentially take another inhibitor. If the enemy team comes to stop you, the Super Minions can transform into Winions and take a Nexus turret.
Finally, it is ok for one of your teammates to die. It happens. People get caught out. Do not, however, end up getting aced trying to save them. Back, play safe, and regroup while you wait for them to respawn. Live to fight another day!