First, get a contract and a payment. This may seem obvious, but I have heard horror stories from newbie speakers and event planners. Don't be afraid to ask for a contract and a deposit. If the other party can't provide this, don't go for it. Too many people have been burned within this industry based on the promise of payment after performance. Contracts are a must and deposits are a must. Never demand payment in full, never accept no payment at all. A 50/50 payment will make both parties involved accountable when it comes to the contract's execution.
You should become affiliate with MPI to avoid any funny business. If you're not speaking at an event sanctioned by a member of MPI, you need to keep your guard up until you become familiar with the client. Not familiar with MPI? Click here for a list of what it is, the types of members you could be working with, and more!
Finally, don't send advanced material without a deposit. There are some meeting planners who like local, cheaper talent. But, this talent might not have the right message. So, the meeting planner has you send your material at no cost (pending a possible job) and then turns that material over to a cheaper, less qualified speaker to present. You should have a video clip on your website- or at least relevant information that will give a planner the basics of your presentation / talk without providing all the materials up front.
Being a public speaker isn't easy. There are a lot of speakers out there, contracts are sparse. You have to protect yourself and your brand from scams. Follow my above steps and you'll be less likely to become the victim of one.