Plus just have fun...keep vent active and joke around. My buddy Olo (our MT) from Sen'jin would always keep the mood light during our 40 man days. Even on our 5 FCKN! hour wipe session on Vael he stayed calm in vent and got us through it. We were the second most progressed guild on Sen'jin (loss of tanks + politics kept us from number one) so we were definitely hardcore, but that guild had a lot of fun and are some of my best memories of WoW.
Now another top guild I was in on Sen'jin had a MUCH harsher RL who also happened to be a MT. Now they got shit done obviously but yelling at people in vent when they are new and have never been in the instance before is just idiotic.
You WILL keep people in your guild, and interested in raiding with a good solid attitude. Stay stern stick to your guns, but don't be a dick. It's easy unless being a dick is just who you are.
Make sure you know the fights.
Know how to get to the fights.
Take your time and make sure you keep in contact with your healers. If they tell you to pull faster then trust them (to an extent).
Don't take all the responibilty in RL'ing. Have a healer/DPS officer help out and have them call things out in vent also. Plan this ahead of time as well.
Also make sure your guild knows that what the RL says is what is going to happen. If you call the raid early then the raid is done. Take control and don't be afraid of it.
Be prepared gear wise. Chants consumables EVERYTHING must be up to par for the raid.
Are you in a casual guild what instance do you plan on doing and is it 10 or 25?