One thing New players need to know about Dominion, is that Runes and Masteries have to be in sync to do well in the mode.
Runes should always consist of Armor per lvl/Hp per lvl yellows, MR per lvl blues, Magic pen/Armor pen/Duel pen reds, and Scaling ad quins/armor pen quins/magic pen quins/movespeed quins/duel pen quins/ armor, magic resist, or hp scaling or flat quins depending on what class you are.
Masteries are as follows
Ranged Ad- 21-0-9(Runic Affinity), 21-9-0(Vet Scars) 22-7-1(Spellblade), 23-6-1(All O)
Bruisers- same as above
Tanks- 9-21(Magic Pen), 0-29-1(Supa Tank), 0-21-9(Runic Affinity)
Ranged Ap- 22-7-1(spellblade), 21-0-9(Runic Affinity), 21-9-0(Vet Scars)
Ap Bruisers- 22-7-1(spellblade), 9-21(Magic Pen/Tanky)
Assassins- 21-9-0(Vet Scars), 22-7-1(Frenzy), 21-0-9(Runic Affinity).
Next thing is item build.
Any physical dmg- Always, Always, Always start pros blade, boots, 2 pots.
Bruisers- go to cleaver, ghostblade, LW, Trinity Force, While adding lifesteal somewhere in between. Boots are choice
Ranged Ad- Always get IE on most ranged ad and LW with lifesteal. Phandom Dancers on vayne and kog.
Assasins- Same route as bruisers
Tanks- should start with Health crystal, boots, Tome, and pots or health/cdr crystal item, boots and pots. Build into Hextech Sweeper/BFT and then get your Aegis'(Recommended) Sunfires, Abyssal scepters, Randiums, Odyn's veil's(late game only), to balance out your stats.
boots are choice
Ap Caster- Pros ring, boots, pots to start. head straight for BFT and then get Wooglets Witchcap/void/abyssal scepter/rylais. must buy a negatron cloak mid game sometime. Armor doesn't need to be as high, because you can kite. boots are choice
Ap Bruiser- Pros ring, boots, pots to start. RoA or Rylais first for HP, then BFT, Wooglets/Wota/void. Must be tanky enough to do your dmg without running out from cleave dmg/poke in a full on engage, so balance out power with tankiness.
One thing to new players, is that Bruisers will win you in a 1v1 fight almost every time. you need to kite if you have a ranged attack/stick with team. Kiting while doing dmg will put the bruiser lower and lower till you can just stop and burst him down. If your exhausted, run from melee range until it's off as you can't trade well with it on.
Your next biggest thing that will save your life, is just map presence. Know where the enemy is and stay in "Safe spots" unless your grouped. You cannot push a lane in dominion unless you have some eyes from your team watching your back. Melee bruisers will grab the speed shrine and gap close you if you have poor map awareness/positioning.
Always have 2 sources of poke no mater what and keep your teams with at least 1 tank/beefy character, 1 hard caster, and the other 2 can be bruiser, assassin, ranged, or anything in between. But most importantly, you need a strong caster to make them build MR so you can do more dmg on your ad champs.
Strategies- 4 top is the general way to go, but ganking bot(Play safe as you will be outnumbered top and will need to stay near tower or in a safe spot), 4 man into the jungle ambush their team(WARNING! do not do vs trap champs) work just as well.
Backdooring doesn't work at High elo, but if you have a person on the enemy team continuely back dooring, have a dedicated person stay with them and follow them(aka anti-pootering). As long as you stick with him, top fight is 3v3 and is doable. It's like "steal the flag," you know what they want, so just stay near and stealthed until they show up. This works for all rammus', shacos, zileans and other champs that build pure movement speed and want to trick you. Don't be tricked and outsmart them.
Summoner Spells- Always 100% Revive, 2nd spell exhaust, ignite, or garrisan. All work well
Once your Runes, Masteries, Build, and Strats are in place, you'll be able to handle just about anything they throw at you and be able to win alot more, because you have the right core for dominion. SR's core runes/masteries/builds(sometimes builds) do not work here and have to be adjusted.