Once you have established yourself as an authority or person of influence in your field, getting paid to speak to organizations or groups is simply a matter of marketing. You have to get your name out in front of the decision makers who control the speakers for various functions. You will need a demonstration video, CD or a copy of a book or a collection of papers. Whatever your particular field is, there are people who want to know what you know that are currently in that field. Getting in front of them takes time and effort, and a lot of mailing, calling or emailing.
For instance, if you are an expert in the field of automotive sales, you will want to gain exposure to car dealerships all over the country. You can arrange lists by state, by city, by zip code or whatever is mist convenient and fits your business plan. Google the names of the decision makers and send them an email with a link to a blog post or a free book you have written. Send them a mailing with promotional materials about you and your business, along with your experience and why you think you will be able to help their salespeople to perform better. This translates to any field of study or line of work.
Most importantly, give those decision makers a compelling reason to hire you to come speak to their group. The best reasons usually impact their bottom line. In the example of the automotive sales speaker, you would not tell the owner of the dealership that you want to help the salespeople make more money. You would tell him that you want to help the sales people to be better salespeople, to bring in more closed sales, to increase HIS bottom line. Marketing is truly everything, and it is the key to obtaining paid speaking engagements.