"Wards win games."
At the beginning of one of my games, the jungler said "wards win games". Surpisingly enough, there was actually a lot of warding from everyone in that game (except for adc, but I don't really expect wards from them anyway). I said that in the beginning of my next 2 games, and again, to my pleasant surprise, people were actually warding a lot more than normal. If you just remind people to ward, it'll be in the back of their mind, but they'll remember to pick up a ward here and there more often. (Also, upgrade yer dang trinkets)

Visions win battles
This item is awesome. I was afraid to buy this upgrade because you can only place one Vision Ward at a time and they otherwise cost a bit of pocket money, but this thing? Awesome. I never feel restricted when I placed wards with this thing, no red sign cautioning that I'm at my limit. They just reveal enemy wards and by the time they're destroyed, I can place another. It's a great investment.