Begin your presentation with no aids, as you want your audience to be listening to you, not looking at props, specimens, or other visual aids (or have the title only).
Present the aid at the appropriate point in your presentation.. Present the aid; give your audience a few seconds to comprehend it, and then comment on the aid.
Turn off/block the projector lamp between slides. Do not begin talking about another topic while a slide, depicting a past topic, is still showing.
Remember: people cannot see and listen at the same time.