There is quite a thread going around with some ideas about how to fix raiding lag. Lag, like many cross-computer issues, is a pretty complicated thing -- there's all kinds of reasons it could be happening, from errors on your computer to errors on Blizzard's end, and all the little connections and switches in between. A certain amount of lag is unavoidable. But there are certainly some things you can do to make sure the link between your client and Blizzard's server is working at its best. This thread, which started on the EJ forums and then moved on to Livejournal, has some good tips in it, including turning off most combat logs like Recount and even Blizzard's official "Everything" log -- having to write down everything happening in game does cost some computer time as you play. Blocking addon "spam" is another way to keep things simple and clear -- while lots of useful addons help you share information between raid members, sending that info back and forth can cause problems when you're down to milliseconds of lag.
The final suggestion is to run a third-party program that's supposed to keep your latency high, but I would be leery of doing that -- a better solution if you continue to have high latency constantly, even after making the changes above, would be to go to Blizzard (and/or your Internet Service Provider, or ISP) with your issues. They have a good guide to smoothing out your connection, and many times the problem can be with your router or firewall, which is usually a quick fix.