Poison TrailPoison Trail is very effective for farming and harassing, allowing Singed to dominate his lane. Poisoning the opposing minions as they advance is an effective strategy for farming lanes. An easy strategy for farming is to push your minions to the turret then run past it to poison the next wave as it comes.
When facing melee champions, you are capable of zoning them by poisoning the area near your own minions so their farming capabilities are hindered severely.
When poisoning the area while being chased, consider how close the enemies are to you, taking gap closers into account, as well as your health. If you can afford to run in a zig zag pattern, do so to make sure your opponents will be poisoned at least once.
Mega AdhesiveMega Adhesive is one of the best slows in the game, and it should have at least one rank before level 5. The upper ranks simply increase the slows percentage. A well-played Singed can lead a target with the glue, catch up to the target, then Fling him back in to the glue for maximum effect. This may take some practice, as all ranks have only a 5 second duration.
Insanity PotionInsanity Potion is useful for chasing enemies and is a useful asset in any major team battle for its all-round bonuses. It has an incredibly short cooldown considering its bonuses, so don't worry about wasting it to run away from a gank gone wrong. Its short cooldown also means it can be used in lane to simply regenerate mana and hp.
Although not a caster in the true sense, Singed benefits greatly from magic penetration runes to make sure your opponents get full damage from Poison TrailPoison Trail. A full caster setup will allow him to deal quite heavy damage with the capability of tanking.
Consider leveling Mega AdhesiveMega Adhesive before Fling, as this spell grants you better zone control and CC, granting you high value of 75% slow, instead of Fling, which grants you only 50 bonus damage per level and increases its mana cost.
Fling must be used wisely when diving a turret, if you are low level and the enemy is on the side of the turret facing your base you can E + W easily to attack, however take into consideration that if you toss the enemy when he moves on the other side of the turret you could possibly land him near to the turret and take lots of damage or land him further into the enemy lane and force your ally/partner to have to walk through the jungle or make his escape easier.
If you are finding yourself running out of mana quickly during laning, buying a Chalice of Harmony is a possible item choice.
Because Singed's gameplay is very dependent on positioning, Boots of Speed should be one of your first major item purchases. However, the next stage of boots is best determined by the enemy team composition.
A harassment/tank Singed should pick up a Force of Nature for the MR and movement speed. The percentage health regen also works well with Singed, as he will get a lot of health just from building Rod of Ages.
An early Philosopher's Stone can help overcome the mana cost of Singed's abilities. Late game, this can be built into a Shurelya's Reverie for additional stats/the movespeed boost. Remember to use it defensively as well as offensively, as nearby teammates will benefit from it as well.
Consider a Will of the Ancients on Singed later on, as the AP/Spellvamp aura also affects nearby allies. During team fights, the massive amount of poison can heal a deceptively large amount on Singed, and a late-game Fling can also restore a portion of your health. And you can also restore a lot of health when escaping with Poison TrailPoison Trail.