Come prepared. Ask experienced trainers for their hot tips. Give relevant and well-presented materials, and the group will trust you; chuck around a few pointers grabbed off the internet, and they won't.
Ease yourself in. Presenting to a group can be nerve-wracking, so set people talking among themselves first. This will break the ice and help you feel less like the second coming of your bumbling old RE teacher.
Pace it. Long lectures will have everyone reaching for the blankets.
People learn by interacting and practising new skills, so balance trainer talking time with practical exercises and regular tea breaks.
Don't hog the limelight. Use anecdotes to demonstrate points, but don't waste time showing off about deals you've brokered or people you've met.
Involve everyone. Group work alters the dynamic, mixes up personalities and gets quieter people to chip in. Invite questions. The more people talk to you, the more they will derive from the sessions.
Get feedback and use it. People will leave feeling valued if you invite their opinion at the end - a simple questionnaire will do. Taking advice and acting on it is also the perfect way for you to improve.