Being a jungler, I run into this situation fairly frequently. You head over to your red buff after clearing the blue side of your jungle and you encounter a Shaco, Lee Sin, or Shyvana clearing your red buff or waiting in the bush beside the red buff to smite steal it and then try to kill you before making their escape.

Now there are ways you can help yourself such as asking your laners to ward the bush beside red at the 1:30-1:45 minute mark and having your ward up* to drop in that bush if it is unwarded or the bush just above red.

But sometimes despite all of that the enemy jungler (sometimes with some of their laners) still shows up to mess with you.

Laners

If your jungler is in trouble and you aren't actively engaged in killing your lane opponent(s) or being killed by your lane oppenent(s) right at that very momenet, drop what you are doing and go help your jungler.

Don't wait to push the minion wave. Pop a health and or a mana potion if you have them and need to, drop a ward in your wake if you have one in order to have vision to detect if your opposing laner(s) decide to follow you, and run, don't walk to where your jungler is probably fighting for their life.

From Blue Side Perspective

If your jungler is on the Red side of the jungle the Bot laners and the Mid Laner should immediately collapse to your jungler's position.

If your jungler is on the Blue side of the jungle the Top laner and the Mid Laner should immediately collapse to your jungler's position.

Obviously check your map and make sure you aren't walking into a 5-vs-3 or a 5-vs-4, but otherwise get over there and help them.

Simply being spotted moving in that direction may save your jungler's life and it sure doesn't hurt if 2 or even three dots on the mini-map start moving in that direction.

Your jungler will love you and you may stop the enemy team from taking an objective or picking up a kill. Even if things go badly for your jungler you may be able to turn a 1 for 0 into a 1 for 1 + potentially assist gold.

Keep an eye on the mini-map and move to assist your teammates when they are in trouble.

Junglers

Show your laners the same level of commraderie. You really can stop farming Krugs if the **** hits the fan Bot lane. If the situation doesn't look hopeless, get in there and turn things around or mop up if the enemy team overstays while on low health.

*At the start of the game I like to place my Stealth Ward at one of the jungle entrances on the side of the jungle where I plan to start my jungle route at the 1 minute mark. This way it will, depending on my jungle route, be off cooldown right about the time I go to take my second buff.