The most important part of jungling is time management.
If you're taking a creep camp then you're not ganking, losing that potential gold / xp
If you're ganking you're not taking camps, losing that ensured gold / xp for potential gold / xp, and also meaning that you'll have to wait longer to farm the camp twice.
If you're covering a lane, you're sacrificing the inflated jungle gold and xp for laner gold and xp.
.. The list goes on, but the point is pretty clear. The jungler makes a sacrifice somewhere to do anything and everything - something which laners only face when they roam. But this comes with the power of unpredictability and freedom.
The two biggest and non-specific tips I can give you are pretty simple, but stuff that a lot of junglers don't seem to have a handle on.
First and foremost, always be active. Whether that's invading the enemy jungle to snipe a camp, farming your own jungle, ganking a lane or cover one doesn't matter. Just always be doing something. Falling asleep in a bush hoping for a gank opportunity is rarely going to yield the results that you want, and will often allow you to fall behind.
Secondly, always gank from unconventional paths. This is a simple trick that will make you much more slipper and much more dangerous. If you're on blue side and ganking bot lane, don't bother with the tri brush. Just walk near raptors, slip into river, and gank from the river. If you're blue and ganking top then cut through the enemy jungle and tri brush, don't bother with the river. This will allow you to bypass early ward spots and (especially at lower levels) start the snowball rolling. And if the enemy wards you out even on these paths then you're controlling where their vision goes, which is good.
Other than those are a few other generalist tips which can vary depending on who you're playing with and what bracket / rank you're in.
1: Never count on your allies to be intelligent and act appropriately. This might be hoping someone will respond to a threat in your jungle, praying that your adc will respond to a dragon attempt after sweeping botlane, or expecting your laner to walk forwards and bait the enemy just as you come into gank. You can't count on your team to be intelligent. If they are then great, enjoy it. Just don't expect it and play accordingly.
2: The inverse, don't expect your enemy to be as incompetent as your team. I realize that these two tips sound really cynical, but in the long run you'll save yourself a lot of grief by just thinking this way.
3: Watch how people play on both sides. If someone seems overly aggressive, take it into consideration. If they're super safe, the same thing.
4: If you see the enemy jungler at the other end of the map doing something, capitalize. This can be as simple as taking the crawler safely or dipping into their jungle and sniping the large mob from a camp. While not the most crippling of blows, playing like this can very quickly stack up and help you mitigate the other jungler making gains elsewhere.
5: Keep in mind that jungling is the most decision making heavy role on the game. You will make the wrong call a lot. Don't dwell on it, but be honest with yourself and learn from your mistakes.
6: It's always the jungler's fault.
