Poison Trail is very effective at farming and harassing, allowing Singed to dominate the flow of whatever lane he's in.
Use Insanity Potion to trick your opponents into chasing you through your Poison Trail.
Flinging enemies into your tower can deal heavy amounts of damage to them.
Keep your distance to avoid being Flung back into Singed's allies.
Singed needs to run close to your team to be effective. Try to take advantage of this by using crowd control effects on him while attacking his allies.
Be careful when chasing Singed. He's very difficult to bring down, and can leave his Poison Trail to damage you in your pursuit.