Runes of Magic US / AU

Learn your classes weakness and strong points.

Choice your fights wisely

Don't play a weak combo

Gear, gear, gear...

Wait.. i suck at this game, ignore everything i said..

Quoted from "kingzamorak"

Learn your classes weakness and strong points.

To dominate in siege this ^ But not just this... Learn EVERY classes weaknesses and strong points, that way you can properly react to each situation you come across depending on your opponent, and not just yourself.

I would also suggest perusing this thread.

What R2P said is probably 33% of what is needed to do well in siege. Experience on all class combos and what they can do and what you can counter with. The other 33% is i guess personal skill/correct decision making during critical moments. The last 33% is definitely gear/fotm class combos. This percentage will increase however if the player is inexperienced or less skilled since OP gear on the right class combo makes rom rom. For the top tier of players I feel it has less impact and understanding of the game mechanics/personal skill comes more into play. A really knowledgeable and skilled player should be able to dominated on w/e class combo they choose to play as shown by various players throughout the game's history of siege as long as the class has potential (obviously d/r mages and r/p will have a harder time to shine than say a m/wd or r/m)

The number one thing that makes this game is gear. so be ready to spend a lot of either time or money to get the gear that is needed to play as a pro. I have watched many excellent players who come in with low level gear as a change of place get wiped out by rather poor players with great gear. Your knowledge and skill are important only if you have the gear that lets you use them without the right gear you will be nothing but fodder for the people with good gear regardless of their skill and knowledge.

I'll just add the age old mantra: practice, practice, practice. OP geared dps can and still do get owned if they don't really have a working knowledge and appreciation of in-game combat. Best way to become better in PVP, raiding, etc. is to do it often. With PVP you have the option to practice outside of siege, arena or battlegrounds through duels. My recommendation is to find players willing to duel you of various classes to get a better appreciation of what they can and will throw at you and learn to counter those abilities.

simple way of becoming pro for pve: level a m/wl and get invited to run. just make sure you crit. if you want to dps well... aint no real ez way. Need good gear with good stats and runes, good class combo, +20 rings, neck weap. easiest to pull off is probably s/r right now with how available stats for it are and the fact you can still get invited for the debuff before you have full gear and the low 2nd class requirement to be viable. for any combo you need to stat 6/0 learn the rotation or if it's too hard for you just use diyce. learn the buff rotation, but if it's too hard can always make a macro
list of other things you need to be a competitive dps: siege titles, zodiac pets, lvl 50 normal pet, maxed out housemaid buff, fairytale event title.

list of strong dps classes: s/r, s/wd, s/w, r/m, wd/s, wd/w, w/s, w/wd, m/w, w/m, ch/r

simple way of becoming op pvp: play k/m, make pvp set mix of stam/int and stam/hp use sod and white hit ppl. you still want +20 weapon, rings, and neck.
Serious talk though, if you want to do good in siege you need stam statted pvp gear regardless of the class you want to play and learn the cooldowns of your class and not forget to use them. that's the basic. If you want to be pro then you need to know all the classes, know where to be. practice your reflexes for things like earth core barrier. That been said just using the right buffs and having the right gear will put you in the top 10% of siege players.
list of strong siege classes: wl/ch, p/k d/wd, wd/d, k/m, k/s, m/wd, w/m, wd/s, wd/w, w/wd, r/m, r/wd, r/k, r/s, s/wd, ch/r, ch/M