Make sure she isn't under any stress, water changes are good...and just be patient. That's the biggest thing with Livebearers. She'll give birth when she's ready. How long has she been pregnant?
You can feed her crushed flake, not large pieces. Fish are opportunistic feeders and will feed when food is available, smaller pieces = more intake. When you feed her, her stomach mass will increase and IF she is ready to 'drop' the pressure that the stomach puts on the 'womb' will help her to 'eject' her fry.
When born and the egg sac has been absorbed, feed fine crushed flake for the fry. Rub some between finger and thumb under water, this creates a suspended paste that hangs in the water and makes easier pickings for babies
Okay, sounds good. For right now, I'd just say patience.
If you do water changes and raise the temperature and keep the stress as low as possible, it could help speed it along a little, but right now I'm thinking it's just waiting. Are you putting her in a fry tank, or doing survival of the fittest for the fry?