Practicing your presentation in front of an empty room is a great way to get to know your own presentation and work out any kinks. But delivering it to someone you trust before the “real” event can be even more valuable.
A second (third, or fourth) opinion can actually be useful at every stage of readying your presentations. Getting a second opinion as early as while you work to define your objective can be immensely helpful. Other people will often spot the errors in our thinking much faster than we can– sometimes we work so long and hard on an idea that we’re too close to it to see where we’ve gone astray. If you’re not very good with design, having a friend who is and can give you feedback on your visual aids can be invaluable; just don’t expect them to do all of your work for you if you want to stay friends.
Even a second set of eyes to proofread your slides and handouts can be very useful. But the most valuable way to get the input of someone whose opinion you value is to do a full run-through of your presentations for them and ask them to tell you what they honestly think– what worked, what didn’t, if you were persuasive, where they got bored. If you tend to say “um” a lot, ask them to count how many time you do it; just knowing that they’re doing this can be a very effective way of encouraging you to stop.
What’s most important here is that you find someone you can trust to give you constructive criticism. Having someone just tell you that you’re doing a great job doesn’t help at all. Having someone who offers bad advice is worse.
Obviously, rehearsing your presentation for an audience is a lot of work and isn’t something you’re going to do for every presentation. But it’s worth the effort for the important ones.