I'm starting to win matches from the jungle a lot more than I used to. The hard part is knowing when you just need to get your farm and when you really should help a lane out. If you're using someone like Jarvan or Amumu, a level 3 gank ins't a bad idea. Even if you don't secure the kill, you can probably get them to burn a summoner's spell or two and also force them out of the lane. That's a big win for your teammate already. Knowing that their flash is down, you can return gank them soon, and keep them in a cycle of always having to play passive.
At your first back, get a sweeper. Start denying vision and making them fear for your ganks. A jungler that puts psychological pressure on the other team is already helping out a lot. Pink wards and sweepers are your best friends.
But really, it comes down to helping fix the trouble spots on your team. It's important that you get your farm, so you can't sit in bushes all day waiting to gank, so just be smart about it. Abuse lanes that get pushed up too far, and then go farm again. Camping isn't the best strategy unless the lane is bad and they really need your help, but a lot of times it's a lost cause with those sorts of teammates.
Note: This is from a fairly inexperienced jungler playing at a low Gold elo. People make mistakes that the plat/diamond guys here don't see anymore, so my advice will vary from theirs.
