The best way to communicate is via /raidshout. /ra is typically jammed up on a screen with a bunch of items being rolled on and many people don't even watch it during a fight. Roll spam can be eliminated by right clicking on the chat tab and unchecking Standard under filters. It's still best though to pass messages right in front of their faces with /raidshout. Voice chat in game is useful in many situations with the limitation being lag. Reports of lagging up to 30 minutes between transmission and reciept of the message are not uncommon. Using a Ventrilo or Teamspeak server is a viable and encouraged alternative but it requires members to download the client. This has the additional advantage of voice communication outside of the raid which can be invaluable in coordinating many small groups or multiple raids.
Keep a tight leash on your bloodthirsty creeps, if you don't, they WILL run off and aggro everything in their bloodlust. No need to yell and berate them, but maintain positive (in both meanings of the word) control over your raid. Designate where we move when we pull, who is pulling what, and where are the boundaries of the raid (ie, stay in the main room! or Move up to the BOTTOM of the stairs but not above!)
Be aware of what is out there against you. Use raid target markers liberally to mark LM's, MIN's, and other major threats. Steer your raid assist targeters as necessary to take out suddenly vulnerable targets or to shift to more dangerous ones. When facing down freeps, it becomes important to break their minstrels especially on Tyrant defense. Often if even the whole raid focusses on a minstrel, if there's 3+ others, they'll be able to keep the main target alive while your raid is destroyed. One way that works pretty well is to split up everyone on their minstrels. Set one targetter (usually weaver) in each raid group as a targetter and have all of your targetters grab a different minstrel for their group. When the time is right, call for the attack and each group now goes after a different minstrel, preventing them from supporting each other.
With multiple targeters there are usually one of three mechanisms working
Keep a trusted creep watching your back from freeps. You can't see 360 while doing everything that you should be doing as a raid leader. You have 24 pairs of eyes in the raid, you can spare a couple to watch your back (BA's do well at this as they're in the back already and can shoot while looking around). When preparing for a raid, wargs are excellent in finding freeps and scouting. Intel is important! GOOD intel is even more vital! Get a headcount on the freeps and the LM's and MIN's identified!
Have a general outline that you present to your raid early on of what you're doing and generally how you're going to do it. Have everyone on the same page. Rebrief the next portion of the plan when you get to that stage. When you get ready to engage the CGs and FM's, make sure everyone knows who is getting attacked and who is getting kited. Designate a lead weaver to coordinate between the webs on nasty NPcs.
Every plan will fail at some point, but it's important to change the plan to adjust. Don't follow a plan till everyone is dead. Voice is key here in talking to the creeps who can hear you (without too much lag), do not get caught up in fighting/healing, make sure your priority is formulating and reformulating plans on the fly to deal with whatever is happening. For the plans that you select, get them out on /raidshout quickly and concisely. Do not be wordy.
Conversely there are those times that you can't execute any plan because of freep resistance or just plain old creep incompetance. There are times to fight.. and there are times to avoid the fight. Don't get stressed over running a bad PuG or fighting an impossible situation. Back off, breath, and say 'Good try all! But they got us on this one!' and pull out. And if there were a couple really bad creeps.. make yourself a note somewhere of their names and make sure they don't get in the raid again. There is being a newbie to the PvMP zone, and there's stupidity. The first you can deal with and make a good creep out of, the second is hopeless.
The number one killer of a raid isn't freeps, it's boredom. If you don't need the raid soon, get rid of it and make it later when you do need it. Don't hang around the outside of Goldie waiting for that last person coming from Grams.. you got 23 people waiting on one... big whooopee, the quest resets in an hour. Go do Goldie and keep the tempo up. I will avoid raid leaders who like to sit around and wait on their friends.