A simple tip for those times when you have things to give away randomly to your audience and forgot your bag of dice...
This site features a Random Sequence Generator which is simple, but effective.
Just have your audience count off, enter the count, then call out the first X numbers based on how many giveaways you have. Because it's a full sequence, you won't get duplicates and you'll have extra numbers if someone leaves or doesn't want a prize.
P.S. Here's an earlier post with five other presentation tips.
The random thing is definitely a good idea. Most presenters ask questions, but that usually results in question that nobody knows or the entire audience blurts out at once.
We've been using raffle tickets for the past few years.