I'm not a very wordy guy, so I'll keep this short. Here's what makes Singed excellent in Dominion.
Pros:
High Mobility: Singed can get where he's needed quickly, ninja unattended control points easily, and generally be a nuisance against an unwary team.

High durability: Though he doesn't do a ton of damage, everything else being equal a good Singed can survive in any 1v1 situation, and in most 1v2 and even 1v3 encounters, making him absolutely invaluable when defending a control point. This durability also makes him excellent at starting engagements under enemy-controlled towers, tower diving for kills, etc.

Cons:
Mediocre damage, no burst: Poison Trail is your main source of damage, and it only works if your opponent keeps fighting you. Your main strength lies in your ability to stay alive and to make it a complete nightmare to try to kill you, not in blowing away your opponents.

Your choices here are pretty limited in terms of what will actually be useful. You could play around with Greater Glyph of Ability Power and Greater Mark of Health Greater Quintessence of Health, I suppose, but in my opinion you're better off this way. You do a moderate amount of damage as is, but it's not your main focus, and you have enough health from your items.

1/21/8 not just because the Defense tree makes you an unstoppable noob-flipping monster, but because the other 2 trees don't have much of interest to you. Again, getting kills isn't your main objective, and Singed doesn't really need CDR, extra experience (on Dominion), or any of that - at least not more than he wants what the Defense tree offers.

In classic, Mercury's Treads should be your boots of choice 95% of the time, but on Dominion I would say that Boots of Swiftness is the better choice about 75% of the time. Why? Because in Dominion the ability to move around the map is more important than the ability to absorb a ton of CC. You're rarely going to be tanking in a large team fight like in Classic, with several slows and stuns being thrown at you repeatedly. Those that you do get hit with, you'll generally be able to shrug off with Insanity Potion. Generally, you should only consider Mercury's Treads if you're facing a team with a heavy amount of CC and/or AP damage.

Boots of Swiftness, on the other hand, give you a huge amount of mobility that will get you where you need to be faster, which is all-important on Dominion. The amount of time you save getting places with these boots adds up and makes a huge difference over the course of a match.

Rod of Ages is still the most important item for Singed in Dominion. No other single item gives Health, Mana (which, for Singed, is also HP), and AP, all of which are vital for Singed. Even though Dominion games are short, you will still receive the full bonuses from RoA by the end of the match (unless one side gets completely stomped, in which case it doesn't really matter). The Catalyst the Protector ability also gives you a nice boost since you're leveling so quickly. Rylai's Crystal Scepter is a possible alternative as a main item, but you WILL be running out of mana pretty consistently if you choose it. Moreover, when you take Singed's passive into account, RoA's base mana gives you an extra 130 HP, or +80 compared to Rylai's.

After you get RoA, though, your build depends on the situation you're facing. Most of the time Force of Nature is a pretty reliable next choice both for the move speed bonus and the hefty amount of MR. If you're facing a particularly AD heavy team, however, Sunfire Aegis, Randuin's Omen, or even Thornmail (if that damn Gangplank or Tryndamere is giving you problems) are viable options. Of these, I typically prefer Sunfire Aegis for the boost to your damage output, and because it's quicker to finish than Randuin's Omen, but each of these works just fine.


If for some reason you find yourself needing even more MR after Force of Nature, an Odyn's Veil will treat you fine. After your indestructibility is assured with these items, or if you're already steamrolling the other team, Rylai's Crystal Scepter is a a solid choice.

You probably have a pretty good idea of how Singed's abilities work. If not, I would recommend checking out this or this. This sequence is pretty standard stuff. Poison Trail is where most of your damage comes from. Mega Adhesive is very useful. Fling is mostly useful for it's effect, not for the damage it does. This skill sequence reflects that.

The only other thing I want to say is that I recommend getting 2 levels of Poison Trail rather than putting 1 into Mega Adhesive at the beginning because during the initial fight at top, Mega Adhesive just isn't very helpful. The fight tends to be very close quarters, as no one wants to move too far away from the tower, meaning that it can't really be used to separate enemies or prevent them from moving to help each other, especially since at level 1 the slow isn't very large. It's also a big drain on your mana to use, and at the start you don't have very much mana to spare at all. So, generally speaking you're better off just leveling Poison Trail an extra time.

Ghost is pretty much the go-to Summoner Spell in Dominion, and Singed benefits from it as much or more as any other champion. It kicks his already high move speed into overdrive, making it very useful in a wide variety of situations.

As for Exhaust, it's a nice choice for tanky characters, and again useful in a lot of different circumstances. Garrison is another potential choice for the defensive minded or aggressive tower divers.

Carrying both Flash and Ghost is probably overkill as far as escape abilities go; if you need both, you're probably playing the wrong character.

Clairvoyance and Cleanse are both meh. Mildly useful, but I can't think of a situation where I'd choose them over Exhaust or Garrison.
Stay away from everything else.

Since you picked up boots first and are fairly tanky to begin with, you should be rushing top at the start of the match. I highly recommend picking up at least a mana potion to start, because you WILL run out of mana in the initial fight. After you get Catalyst the Protector you'll be fine mana-wise, but if you run out at the beginning you're basically useless.

Mid and late game, do not be afraid to throw yourself into ridiculous situations, especially if Insanity Potion is up. Ninja unattended capture points, run headlong into defended ones, defend against 2 or 3 at a time. Even if you don't win, you can buy enough time for your team to capitalize on the distraction you've created.
Singed is one of the few characters in the game that can throw himself into most situations with abandon and come out alive, or at least with a kill or a capture out of it. For that reason, he's one of the funnest champions in the game. Enjoy!