While you're talking softly to the cat (I actually whisper), look her in the eye and SLOWLY blink your eyes. This will let her know that you mean no harm. Reassure her that it is okay and you will not hurt her. If she blinks slowly back at you, that is a sign that she is becoming more relaxed. Also, when you sit on the floor, let her come to you.
Another thing that I try when socializing a feral (especially kittens) is to wrap them up in a towel with only their faces exposed. Sit on the floor with them in your arms, whispering and slowly blinking your eyes. Pet them on the nose or the top of the head.
I hope this helps.