Mage
From Twilight Wiki
One of the most important things as a Mage is casting. If you are not constantly casting you are loosing out on DPS. If you start to cast a firebolt, frostbolt or anything else that isn't your main nuke, it is better to let that finish casting then to cancel it and start casting your primary spell.
A great add on for casting is Quartz. The best feature is it accounts for lag between your PC and the server and represents it by a red highlight on your caster bar. If you cast cast your next spell during that time the server should reject the cast and give an error message. This is a great tool for improving your dps.
You can find the add on here http://wow.curse.com/downloads/wow-addons/details/quartz.aspx
Frost
One of the main benifits of Frost with Raid DPS is Cold Snap. If you have one or two trinkets, during Heroism or Bloodlust you can pop one trinket and Icy Veins. This will bring your cast time for a frost bolt down to about 1.5 seconds. You can then use Cold snap right away and when Icy Veins is done, use your other trinket and Icy Veins again. This is an incredible boost to your DPS for a boss fight.
Fire
One of the biggest things about Fire/Frostfire build is the rotation. One of the main components of increased DPS is Living Bomb, keep it up at all times. Then use the scorch debuff and fireball/frostfire until either needs refreshing.
Also, using your mirror images is another increase. I find it best to use them when AOEs are being cast. That way they can keep DPSing the Boss while the real Mage handles the running and living.
Just some info I found helpful. :) - Ave