When they get within arms length, press the X button while holding the aim button. If it's a zombie, Jill will push a them. If it's anything else, she'll just dodge to the side.
I do just that! Hold R1 and push X like you're gonna attack and you dodge. I just push the poop outta X and I dodge all the time.
Plus you can bait zombies in a certain direction then run the other way around them.