Know your subject matter well.
Be enthusiastic and sincere in your teaching.
Gear your presentation to the skill level of your students and individualize the instruction as much as possible. A one-on-one in the welding booth with a quick sketch on the welding bench with a piece of soapstone almost always trumps even the best Power Point presentation.
Look them in the eye when you're done. If the light didn't come on, you did it wrong.
Pretty much all there is to it.