Taking Flash will help with the positioning for your ult to separate their APC or ADC from the rest of their team in team fighting resulting in an easy kill, or simply using Flash to escape or catch up to an enemy.

Smite, typical jungling summoner spell, necessary for early game camp clearing.

Items all depend on the team comp of both teams. For instance, your team consists of an Akali top, Malzahar mid, Sona support, and Ezreal adc, you probably should go with an AD based jungler, preferably a more tanky jungler, like Volibear, for the AD and the tankiness.
If their team consists of Talon mid, Vi top, Ashe adc, Voli support, and Nocturne jungle, you should probably go an armor heavy build.

For my Runes, they're pretty standard for an AD Bruiser Jungler. Armor Pen. Marks, Flat Armor Seals,
MR per level Glyphs, and Flat AD Quints. That's the optimum amount of defenses and damage for an AD Bruiser Jungler.

For my Masteries, I like to go 16/14/0 for optimum tankiness in the jungle, and good enough clear time.

You'd be surprised at how many times your Masteries have saved you. How many times have you barely survived an Ignite with just 5 health? If you didn't have those defensive masteries, you probably would have died! Masteries, no matter how little they do, make huge changes in your game.

You'll want to get your "E" first, unless going for a level 1 invade, then you'll want to get your "Q". Getting your "E" first will give you the AOE jungle camp clear, and it will reduce the monster's attack speed, resulting in you taking less damage. If you plan to go for a level 2 gank, then get your "Q" second, if you will keep jungling till at least level 3, get your "W" before your "Q".
Maxing your "Q" first will give you the damage you'll want, but if your team doesn't have much CC, for instance; Nidalee support, Katarina mid, Sivir adc, and Fiora top, that's not very much CC! So, maxing your "E" in that situation would be the best, to increase the slow of your "E".

The Pros to a Lee Sin jungle are:
Great early game ganks
The positioning to pick off certain targets
Being able to stick with an enemy while they're running
Great escapes, hard to catch
No mana
Good counter jungling
Good early game damage
Nice sustain
Doesn't need Blue, can give imediately to mid/top
Good CC's
Good at stealing buffs
Can see invisible targets like Shaco, Rengar, Akali, etc. with his "E" Which is great against invisible junglers.

Cons:
Squishy early game
Not too great late game
He doesn't have the best clear time

The Pro's out weigh the Con's, so all in all, I'd say Lee Sin is a great jungle choice, in most circumstances.