Always buy wards with excess gold. As long as you have a slot and ~90-300 gold: buy wards. If you have to wait 3 seconds in base for a ward, do it.
Some champions should favor wards over others. Champions with high map mobility are more likely to return to base, and should purchase more wards. Support heroes have less need for tons of gold, they should buy more wards. Other champions should follow the "extra gold" rule. Junglers should especially be buying wards through mid game and should be the primary person to ward crucial river spots due to how often they move around the map, minimizing EXP/gold loss.
Place wards when you know people aren't around, or you are passing through an area.
Also, one of the most important ward placements not mentioned: the back of Baron's den, which allows Purple team to kill Baron without doing so much as going through river, and allows Blue to see this. Other important ward spots are any checkpoints that you have to pass through to get to Baron, if you are about to take him. Bring 2 wards and put them in the bushes along the way to Baron so that if a gank is coming you are prepared for it with optimal positioning. Baron changes games, 200 gold often does not. Buy those wards for extra security. A safe baron kill or acing the enemy team will go a lot further than the mild inconvenience of gold loss.
The #1 way of making it into higher Elos is buying wards and playing a champion that can carry a team in some way as well as good leadership abilities.