I have a hard time figuring out how to counter him...

The problem is:
-you must not follow him so he can engage/disengage whenever he wants
-damage is aoe so in teamfight it has similiar damage to ap carry
-if you ignore him and focus carries you die to his poison, if you focus him his carries kill you

Naturally, there is the usual THEORY to try to outdamage their team get resist and on hit slows like frozen mallet and madred`s bloodrazor... but the problem is that those items are expensive and usually (always in my experiance) a good Singed will force out a victory before he falls off in late game due to enormous dmg and countering him with items.

I am asking you:
a) which champions counter him?
b) what tactic to use... play def/off, avoid decisive battles, force decisive battles, etc
c) wtf works in practice... I am fed with theory... I know what should give you the advantage... what did/does work in practice against a good Singed player?

i'd say skarner and nunu work really well against him. constant slows are a problem for him. he doesn't do much in teamfights late game so just try your best to ignore him and burst down the rest of the team

Nunu has worked well for me against singed. also if you can shut him down early with a ranged champion he will be generally useless.

Try a ranged carry.

Lulu works well for me, too. He can't get close to my team because of slow and speed boost, and gets kited hard.

If the game goes late too some extent you can just ignore him and go for damage dealers. This depends on team make ups and such. What I've seen in late games is that singed is super hard to kill, but he also can't do enough damage so you can kill his 2 teammates then move on to towers.

Depends on teams tho.

anything that deals damage..

personally i have had alot of success with Darius the right runes and build setup. he wins the lane and in team fights build tanky with the right team comp can disregard singed. and kill the other enemy's if signed chooses to use his utility on Darius your team can freely attack the other team.

Amumu generally will beat a Singed of equal quality.

Singed's damage scales well vs soft targets, but does so at great expense to his survivability. Any champ that can lock him down while lifestealing will beat him, as will any champ that can mitigate the poison while doing other things.

For example, Jax can wreck a singed. Seriously though, amumu is the way to go. Get him some spell vamp, MR, and about 200ap and watch singe melt faster than you do, all while you lock down his teammates with your ult and q.

Just get somebody with early/mid game damage. :P And snowball the lane.