DISCLAIMER: This guide was made early 2010. Items, champions, summoner spells, and the metagame have changed since then. Take this fact into account.
Singed is the #1 most fun champion to play in this game, period. When played right, not only is he a blast, but he's also a deadly tank that's very useful and nearly impossible to kill.
Amazing for griefing, amazing for farming, amazing overall. If your lane enemies can't grief you a massive amount, start farming with Trail right away. For killing later on, try not to stay near the enemy champion. Instead, run ahead of them with the Trail to force them to run through it in trying to get away. Also, use Trail for run-away griefing if you can spare the mana.
Great for chasing and helping you or an ally(allies) get away. Try to land it just barely past the enemy (if chasing) to force them to run through it for an extended period.
Most fun skill in the entire game. You must learn to use this correctly and not end up like an Alistar that headbutts enemies away. Once mastered, Fling is deadly fun.
I tend to save this only for chasing or, more frequently, getting away. If you're needing to get a turtle away from the turret, you can pop Potion, Adhesive them, and Fling them out before they have time to do anything.
Due to RoA being Singed's best friend, farming is vital to Singed. A shut down Singed is a generally useless Singed later.
Farm the heck out of your lane with Poison Trail (if safe). Smile with glee if your lane enemies are melee and not Blitz or Alistar, because farming will be even easier. Don't waste your little mana on Fling unless you are sure you and your lane-mate can kill the target.
Items: Catalyst, Boots of Swiftness, and possibly the first Rod of Ages.
You should be close to or already have a Rod of Ages. You're one step closer to being an amazing tank. Keep on farming up for the second Rod of Ages, being sure to be the initiator for team fights and annoying the heck out of enemies with Poison Trail.
Items: (Rod of Ages #1), Rod of Ages #2, Guardian Angel, possibly Rod of Ages #3.
You are now the dumbest thing seen in this entire game: an unkillable speed machine of pure evil. Don't be afraid to suicide in to Fling an enemy out of a group or from a tower, unless they can stun you like crazy. Run ahead of the enemies, laughing while they slowly try to get away while you poison them to heck and Fling them back to your team constantly. Enjoy being the most fun character in this game!
Items: (Rod of Ages #3), Force of Nature.