early game 2 wards, 1 in the middle of the river on both sides or in the bushs at the edge of the river means your not relying on mia calls now since you will see most ganks coming a mile away, in solo que this is a way better option then relying on randoms for your survival.
mid game place them at all the buffs and the dragon, teamwork is needed here since 1 person buying all the wards is gonna hamper the whole team harder then everyone chiping in. feel free to place wards in high traffic area's aswell, can pick of straglers very fast to make your money back or steal buffs and gank the fool trying to get it.
late game you want to do exactly the same as mid game but more emphasis on high traffic area's since killing a stragler will make the fight 4 vs 5 and respawn is long at this point, also have a vison ward at the baron at all times(the one that can detect stealthers to kill their ward if they try to place one aswell)
wards are easily hands down the best item in this game, shame so many people dont use them.