Shot calling! It's super important. Play around baron/elder dragon late game, keep them both warded, don't push onto the enemy side of the map unless your whole team is there (they don't have to be in the same lane, just don't push out while someone is in base or farming near it). If you are not in voice chat with your entire team, ping before engages, ping your overextended allies to retreat (don't spam ping them though), and spam ping baron/dragon/tower/inhib after a successful teamfight.
Just use chat/pings to make sure everyone is on the same page with playing out the late game. In laning phase make sure you keep track of summoner spell cooldowns (type when an enemy burns flash/TP, have your jungler priortize ganking a lane who is flashless), and the position of the enemy jungler. You can ping out the enemy jungler whenever he steps on a ward, in case one of your allies doesn't notice him crossing it.
This advice is most important in solo queue, but if you and your friends already good at communication just make sure you include any players on your team who are not in voice chat with you.