This guide is mainly for more advanced players, but can also be used as a comprehensive guide for new players. Enjoy :)

If you do not usually jungle, Elise jungle might not be the character you should be playing only for the reason she is a more advanced jungle character.
Jungling can be difficult if you aren't using your abilities right.

First off you want to start Blue. Use up your Violate Spiderling, then ult into your spider form and start Skittering Frenzy. Like I said, if you don't use your abilities in correct orders, it could be fatal to this character.

Always be aware of over extending lanes.

When ganking in a lane, start off in human form. You usually want to start ganking around level 3 or 4 if you get the chance. In human form, pop off an -E- and use up all your spells in human form. Then, go spider, use up your Rappel to jump to them, hit a Venemous Bite, and then use your Skittering Frenzy.

Magic Penatration: For Elise you really want to be able to hit some hard AOE and assassinate targets fast. Magic Pen. will help you do that.

Immediate Ability Power: Since you are jungling you need a bit of AP to help start you off, and it really helps with the gank giving that extra damage that is needed.

These masteries help to deal a lot of damage and make sure you have some mana sustain.

People build Elise almost purely AP, forgetting she has a melee counter part with Attack speed spells.

Sorcerer's Shoes: Kind of obvious. Increases Magic Penetration and Movement Speed

Hextech Gunblade: This item is very important. It gives Ability Power (for damage on spells), + Attack Damage and Life Steal (for your spider form, it gives LS sustain and extra damage). It also has a nice Active: Dealing 300 Magic Damage and slows the target hit.

Lich Bane: Increases Ability Power and Magic Resistance. Has a wonderful Passive: When you use an ability, your next physical attack will deal 100% of your ability power in damage. Very important item for your build.

Rylai's Crystal Scepter and Rod of Ages: Both for same purpose basically, more Health for sustain and Ability Power for spell damage. Also, Rylai's Crystal Scepter has a unique Passive: Your spell damage will slow the targets movement speed by 35% for 1.5 seconds (15% for AOE spells), which is nice for chases and team fights.

Rabadon's Deathcap: This item just enhances and really intensifies the build. It will boost Ability Power, making Lich Bane capable of doing much more damage, and for increased spell damage.

Smite: Used for jungling and other creep related purposes. Use mainly for Blue and Red buff, Dragon, and Baron. I also like to use it to quickly grab golems when I'm at a low level (usually form 1-7) before I head back to base.

Exhaust: Used for ganking in lanes. First you want to pop off spells in human form, then Repel and Exhaust them OR, if you feel more comfortable doing this, Repel to them in spider form, Exhaust, then go human form and pop off spells.

Pros:

Great Damage Output
Very Good Ganks
AOE Slows in Team Fights
Good Assassinations
Good Health

Cons:

Not the Greatest Sustain
Usually Focused in Team Fights
Early Jungling can get a bit rough.