Examples include people for keynote presentations and panel discussions. This is for cases where you don't have any direct or indirect connections to the people.
The easy answer is to give them something that they want. Although this could be money, it could also be access to your audience. Find out what they are interested in achieving with their speaking engagements, and then make that available to them.
Are they a spokesperson for cancer research? Agree to donate a portion of your conference fees to cancer research. You get the idea - find something they are passionate about, and help them.