Jungling takes practice - a lot of practice, and I still don't have the major season changes figured out just yet. For me, one of the biggest things is knowing what to do when duty calls - is it worth your time to gank, or are they in a favorable situation? For example, if your Shen in the top lane has been pushed under his turret by Nasus, he's perfectly fine - tell the Shen to grow a pair and taunt Nasus so doge draws turret aggro.
also, don't gank a lane on the other side of the map if you aren't going to hit up monster camps along the way.
another thing is to keep track of where the enemy jungler is at all times (assuming there is one). If I see Amumu ganking the lane from his Golem (blue) buff, that gives me free reign to throw down a ward at the camp, and if it's there, I can call my mid laner over to steal the buff, which will help him/her big time.
My big thing about jungling is map awareness. Everyone I play with has the map awareness of a goldfish and only cares about themselves. If you're aware of what's going on everywhere as opposed to what directly impacts you, then that will set you apart, especially if you're playing a jungler with global presence (ex. Karthus or Shen, but those guys are hard junglers to play as).