Before you start decide what type of group you want to lead. It's good to have an idea of what areas you are going to visit in advance, although this plan can sometimes change for example if your group size changes or your group are not getting decent xp.
Choosing the right area for your groups strength is critical, a change of one or two members can a big effect on the type of areas you able to run. Also make sure you adapt as the group grows and shrink by changing to more appropriate areas.
Have at least a rough idea of what each members role is in your group. It is not always necessary to assign roles, experienced players will usually know what to do. Regularly monitor the group and make sure everyone is doing what they are suppose to.
Communicate with your followers give them clear and concise instructions on what to do.
xp/time is a far more important stat than whether a mob caps or not. Although it can reassuring to see a mob after mob cap or even overcap however if the mobs take a long time to kill then the overall xp gained can be less than fast killing a lot of half cap mobs.
For pure xp runs it is sometimes best to avoid fame mobs as these type of mobs are usually harder and take longer to kill however they also typically give more xp, so if your group is strong you may like to target ones that you know won't take too long to kill.
As a general rule of thumb the more dangerous an area the more xp it will provide, the trick is to find the right balance and walk the thin line between been challenged and the whole group dying.
Mana management is an important part of any group. Be sure to monitor your groups mana especially the healers.
Having to regen wastes time that could be better spent killing mobs and getting xp, a good leaders will limit the amount of regens and only order it if there is a reason to do so.
Don't be afraid of booting members who are proving detrimental or not contributing to the group objectives. look out for idle players who are leaching xp. if a player is slow to respond don't be afraid to leave them behind, they can always rejoin once they have got their act together.