Pace yourself. Break down your presentation into small, manageable chunks, and work your way through each chuck, bit-by-bit. I like to break down my speeches into 2 minute sections. Once I have completed a section, I forget about it, and move on to the next section. This helps me keep calm and keep the pace throughout a longer talk. It can be hard to keep calm for an entire hour presentation, but it is easy to keep calm for 2 minutes at a time.