Singed is a great character and one who is more traditionally played as a tank, however in this build ill explore the usage AP singed a surprisingly effective and in my opinion almost always underestimated.

Pros:
great CC
High HP
Great magic damage output
Godly Farming
Will probably get you laid
amazing 3v3

Cons:
Not an actual tank
You won't be able to push all the ladies off you once they see your LoL prowess

With the current build your health will be somewhere around 4k and 800+AP

Runes: Cooldown reduction blues

Masteries: if you want flash take Blink of an Eye

Items: Force of Nature is a good alternative to archangels staff if they have alot of AP and I get it constantly- or of course there is Banshee's Veil if the enemy team is CC heavy and then Abyssal Mask is also a good choice, if its AD heavy-- Zhonya's Hourglass is the way to go.

Now of course this all recommended on the assumption your not a troll, now if you are Phantom Dancer x5 with Mobility Boots is where its at...

Fairly strait forward
-- Poison Trail is your primarily leveled ability for farming
-- Fling at level one on the off chance you can toss somebody into the tower
-- Fling is taken over Mega Adhesive due to its nuke capabilities ( its perfectly acceptable to take Adhesive if you find this is just not your cup of tea)
-- Insanity Potion Makes you tanky and an awesome chaser

This is one of your bright spots, with your lane partner ( or if your 1v1 top) push them back to their tower and then run infront of the minions on their way to the middle while laying down your poison. Congrats you just killed them all and now your minions are running into their tower denying them gold and deal tower damage. A ward is recommended so you don't get ganked. People tend to run away when you throw Mega Adhesive under them so you can use that to juke them or in the case of a real gank just run up and toss em back to your Udyr or whatever.

Ignite: finishing people off. (its not like that, get your mind out of the gutter)
Ghost: chasing and escaping-- nuff said

Team Fights:
Get your poison on and go up there and tap dat Ashe, use Mega Adhesive so she can't get away. She was totally asking for it anways, did you see what she was wearing? Now that she is tossed back into the bulk of your group as well as your poison she is as good as dead. speaking of which so is everyone else that has been chasing you through this poison. Your massive amount of health coupled with your ult make you very very hard to kill and if they do get you low they are prone to try and follow you down. And chasing AP singed is a very very bad idea...

Overview:
toss squishies to the horde
Poison people
Slow people
Profit ( this is no Madoff scheme!)