The first three items I buy for singed never vary. Order may vary depending on what kind of luck I am having dealing with my lane. After that it depends on the team you are playing what you should go with. Heavy magical damage means you should get Banshee's Veil and/or Force of Nature. Lots of snaring from ranged carries implies a need for Warden's mail. A lot of physical DPS calls for thornmail.
The items recommended imply that you are facing a well mixed enemy team, which does make your life slightly harder.
a few notes:
A. Between ghost and your ultimate there should be almost nothing you cannot hunt down. Similarly, you can run away from almost anything. If you find yourself being snared when you try to run away by ranged DPS with those silly slowing items make Warden's Mail a priority.
B. Infinite growth potential. Archangel's staff has excellent synergy with Singed. Similarly, Rod of Ages will keep your stats getting pumped through the mid game. There have been a couple of instances in super long games where this makes all the difference. With your first three items done there is no reason not to spam all of your abilities except the ult at any opportunity.
C. Initiate. Always initiate. That is what singed does. Getting a toss off to separate even one enemy from the rest of them can indeed be worth your life. All of the damage being spammed on you isn't being spammed on your team. Though, frankly, I don't die that often.
D. Early game you should be pushing the enemy back into their tower. When this occurs you can situate yourself between the two enemy towers. Don't do this for too long because it may attract negative attention from other lanes, but use it as an opportunity to gas enemy reinforcements before they even touch yours. You are going to want it clear of enemy creeps when you and your ally decide to toss and murder the tower hugging enemy pos.
E. Don't underestimate how painful your gas is. Remember that your toss does a helluva lot of damage as well. Don't be afraid to throw goo at the enemy when there are no plans to engage. It is annoying to be slow and you are only getting more powerful.