Compensate and pay expenses for your speakers
Preparing a conference talk is a lot of work. If you don’t pay your speakers, you’re restricting yourself to people who can afford to do that preparation for free.
Ideally, you should be compensating speakers for their preparation time, travel, accommodation, and time away from home. You probably want to throw in a free ticket as well, so they can hang around after their session is over.
Whatever your speaker package is (even if you can’t pay anything), be upfront about it in your call for proposals. If somebody can’t or won’t speak at your conference without compensation, it’s better to know that before they submit. Don’t waste time getting proposals that have to be withdrawn later.
(Python conferences are especially bad at this, and it’s something I feel acutely guilty about as a PyCon organiser.)
As with financial assistance, remember that not every speaker may be able to pay expenses out of their own pocket, and be reimbursed later. Think about if/how you can pay advances before the event starts.