As others have said, your best bet is to pair them up with a type of anemone they'd be hosting in the wild. For ocellaris, that's a Magnifica or Gigantea. Even then, it's never an absolute given, especially since they're captive bred. It should help your chances though. Some folks have had luck with getting them to host other types of anemones, but it's really hit or miss. Good luck!
I'm just making a friendly correction here, there actually is pretty much a sure thing when you give a natural host match anemone, I see this all the time, clowns refused BTA's for months, then given a mag or gig and they dive straight in, and this goes for both wild caught or captive bred, that matters not.