I believe Singed is meant to be a very fast and evasive person so that he can catch his prey and toss them to the wolves(his fellow champions). Early game he is slow, but faster than most with boots and his auto attack hurt quite a bit. Use this advantage to either toss enemies into your tower as they try and get away or play aggressive and if you have a high damage output teammate in your lane. His mid game get more based on evasiveness. Singed needs to be able to go into battle and be able to run out with enemies trailing him just out of reach of him, while in his poison. Late game you should use his skills to pick off the weak in team fights. He can run into enemy towers and toss and foe into his teammates waiting just out of reach of the towers fire. With the boots and FoN his movement speed is high, and if you feel they are catching up, you still have your ghost and ult to increase your speed for a short period.

Marks - The magic penetration is very helpful for your skill damage. You can also use magic resists marks, but I feel you wont need magic resists due to the items that you buy do a good job of supplying that.

Seals - I personally enjoyed mana regen seals, but my friend Sup Crown suggested that i should switch it for hp regen and get mana regen glyphs. The hp regen is so vital for singed's survivability.

Glyphs - You need mana regen for singed early and mid game, but do to the items it is not top priority. That is why i switched my mana regen from seals to glyphs.

Quintessence - The movement speed is very helpful for early game and it puts you at great speeds mid game. You can also use flat health quints. Gives good health and survivability early game.

Recommended=*
Items
Boots of Swiftness*
Catalyst the Protector*
Rod of Ages*
Rylai's Crystal Scepter
Force of Nature*
Banshee's Veil
Sunfire Cape
Randuin's Omen
Frozen Heart
Thornmail(If they have a high dps)
Warmog's Armor and Guardian's Angel are fun, but not necessary.

Never Use
Tear of the Goddess or anything that builds from it.
Beginning game i would buy boots and 2 health pots. you can get 3 but i feel i don't need the last one. the boots of speed give that edge to catch champs and do big damage. By level 6 you should have boots of swiftness and should have/almost have your Catalyst the Protector. At 20 minutes, it is almost vital you have your RoA(Rod of Ages). Because every minute is helpful, so sooner the better.:)