I have a few thoughts on that topic:
What exactly is a 'premade'? Do you consider two players who know each other well and have played a lot of games together without using VoIP a premade? What if it is a 3v3, 4v4 or a 5v5 and they play with 1, 2 or 3 random players? Still a premade? What if I know one of my teammates is a bad ass player because he just wiped the floor with me in the previous game? Do I have to play on the other team now to even the odds (maybe, maybe not)?
I sometimes play in random custom games. What I see all the time are 3 people working together and 3 Conans running into every 1v2 or 1v3 fight they can find. Or people who try to get a kill instead of supporting a teammate.
- If your teammate limps away with 10% health and at half speed f-ing shield/heal him or stun the chasing opponent or drop some mines.
- Stick together, attack together! I played a Prison game where our Oak was killing creeps 5 steps behind me and kept doing so when I engaged the two closing enemies. Imagine my surprise! No support, no Penitence, no Shield, he kept killing creeps! (It was his 3rd game online, still...)
If you see the other team ganking a single player help him out! Doesn't matter if you know him or not. That's what parts good players from bad players and winners from loosers.
- Use ping. You are standing in the middle of the map and you don't know what to do? Spam pings (point mouse and press 'P'). You don't need to chat or discuss strategy in most cases, a good player knows what to do.
- Anticipate situations. When you see two opponents going to the lane where your Rook is all alone try to get there in time. Good players keep an eye on the whole map and make educated guesses where the other team might show up in a few seconds, try to do the same. If you can't buy Totem of Revelation - the most underused item I know of - and place it at key positions on the map. It will also reveal Regulus mines! It costs 50 gold and there is no reason not to have 2 or 3 on you in early game if you need them.
- If the other team is pushing you away from flags and portals constantly buy flag locks and use them. 'Ninja' a flag back, lock it, rinse and repeat.
Finally, if you know there is no way in hell you can turn this game around talk to your teammates and concede. But wait until you are absolutely sure you are going to lose. I have been on both sides and I don't enjoy sloughtering new players or being sloughtered at all. The best games I had were against superior teams, they were long, bloody and each encounter a fight with fangs and claws.
If you know you don't stand a chance offer to concede. If you are on the winning team don't drag the game out to get more kills or demoralize your opponents. Make it quick and mercyful. And don't farm the AI when somebody quits, it's just sad. Finish them off if they are in your way but don't go hunting them. Nuff said.