PSA: Shove the minion wave into the enemy tower after a kill.
MAIN POST:
This will primarily apply to lower elo but you'd be surprised how many junglers still don't know they have to do this even in Diamond (As of last night, I'm currently D1 54 LP this season in my main account, so I literally get to play with everybody from Plat 1 to Master/lower Challenger).
This is primarily from a Solo Queue jungler's perspective, but applies to every role.
When I'm ganking, I'm not just looking to get the kill. I'm looking to deny the enemy laner as much gold and XP as I can. After a successful gank and surviving whatever follow-up from the enemy team, I will always tell (aka order) my own laner to push and help him/her shove the wave to the enemy tower. If my laner died, I will shove the wave myself even if he/she pings me a million times not to "steal" his/her CS (CS which they can't get anyway, seeing as they're dead).
This is so that the enemy tower will kill our minions and deny that CS from the enemy. The enemy might have still gotten them if we didn't push the wave. Doing this constantly in the early game will eventually get my laner ahead of the enemy laner in gold and levels, as well as priming the towers for taking with some good damage. Plus, this can free up your laner to roam also and make plays on the map.
The thing about League is that even if you're super fed, it is still gonna be difficult to carry a game if your laners are behind. So to make your job as the jungler easier, your #1 priority as the jungler is to make sure that the laners are doing what they're supposed to, winning their lanes and taking towers. In return, they will help you with taking objectives that will ensure you win the game.
EDIT - SIDE NOTE:
Similarly, if you as a laner have won your lane, always be looking for ways to extend your lead to the other lanes. If you're a top laner, shove your wave then teleport to bot lane and help them shove theirs. If you're a midlaner, roam to the side lanes and gank when your jungler can't. If you're the ADC or support, take your tower then rotate to another lane and shove the wave there too.
When all the great players say that League of Legends is a team game, they mean that every action you take should at least be geared towards helping your teammates in some capacity. I would rather go 0-0 and have three 10-5 teammates than be the 30-0 carry with three 0-5 teammates.