Some of the best suggestions:
If you know that the enemy has a slower clear-speed, get your team to help you kill blue without using smite, and then immediately head to the enemy red and take that (using smite). You can then wait for him and gank him, or go and gank a lane with double-buff.
Similarly, know the timers for the buffs - most junglers start at Blue, so you can expect the enemy's Blue to be back up at about 7:05-7:10. If he is in another lane you can be prepared and take it before he gets there.
In general, if you have the time grab some of his wraiths or wolves, but overall you are better off looking for ganks, or to think about where the enemy might gank, and go there yourself (to countergank)
As a final tip when counterjungling, to have the most effect you should only kill the large minion from the camp (or at most all but one small). This way it will not reset the timer on the camp, and the enemy jungler will be further denied exp/gold/buffs.