Start by proving that you’re a decent speaker. Make a video that shows that you can present an informative, intelligent, and entertaining talk. Then post it on YouTube and promote it a little. This gives you social proof, and supports your application to speak.
You can also reach out to satisfied clients and ask for recommendations. If they attend industry conventions, they may have the opportunity to tell the event organizers that they want you to give a presentation at next year’s conference.
Before you hit the big convention circuit, you might want to build your speaking résumé by presenting at smaller or regional events. There are many industry or topical groups, such as Java User Groups (JUGs) and Linux User Groups (LUGs), that actively seek people to give short presentations at monthly local meetings.
Some conferences take public requests for speaker pitches. This usually has to be done far in advance of the actual conference date. To speak at Search Marketing Expo (SMX), for instance, go to the SMX website and fill out the pitch form. SXSW has a panel picker on its site.
Some conferences do not take open requests. If that’s the case, contact the conference organizer and give your pitch directly.