Assuming you are facing an enhancement shaman:
It's all about kiting...as a warlock you want to dot the shaman up and stay away from him/her. Let your dots slowly kill him while you run out of range. If you are in melee range of an enhancement shaman, be prepared to take a lot of damage especially if you get WF-critted. With the new 3.0 patch, enhancement shamans got a really nice buff to DPS...so definitely watch out.
Key shaman mechanics that you need to be aware of:
1) Shaman totems provide nice buffs but they are immobile. Enhancement shamans need to be in melee range to kill you...so they have to chase you around. Dot them up and run away thus pulling them away from their valuable totems. If he keep dropping totems...keep running away...sooner or later he will be out of mana
2) All shocks (frost, flame, earth) share the same CD. This shared CD is a huge advantage for you as a warlock. Assuming you dot and run away, most shaman will frost shock in an attempt to slow you down so that they can get in melee range. At this point you can turn around and fear since there is no possible way of getting interrupted by an earth shock. This next point is extremely important: Create a macro that will have your pet attack totems especially tremor, earthbind, and grounding. A smart shaman will drop a tremor totem the instant they see you casting fear --> stop casting, have pet attack the totem, re-cast fear.
Elemental shamans have the same mechanics as above but prefer to blast you from afar. This is where you take advantage of your felhunter's spell lock. Be sure to use it when he is casting a spell - this will give him a 6 second (instead of a 3 second) silence from the respective spell tree.
*****If available, Curse of Exhaustion is your best friend. This spell is a deal breaker in your favor - Use against every and all shaman.
Naturally, not everything goes according to plan, but at least you can adapt your style to hopefully win more than you lose. Really good dueling shaman know their CD's and timings by heart and do a really excellent job at minimizing the class weaknesses. If you know the same then you know the windows of opportunity to attack.
Edited, Oct 28th 2008 3:57pm by justoneguy
Edited, Oct 28th 2008 4:02pm by justoneguy