No matter what I say or do , I cannot seem to gain respect from my 12 year old stepdaughter. I try to be nice , offering help with homework , making her special snacks ,etc. Though I try everything I can to be a role model ,without overstepping boundaries, she is okay for a while and then reverts back to her "old self" and ignores me , disrepects me ,and is pretty adamant at showing me that she doesn't need me for anything unless she wants something or asks to go somewhere. She knows how to "play" me, and goes out of her way to do every little thing she knows that will annoy me , knowing I will get frustrated and/or angry. I'm at my wits end and I can't figure it out

Get her involved in making snacks with you and find other things the two of you can do together at home or elsewhere. Does she know how to sew on a button? Knit or crochet? (lots of neat things for 'tweeners to make)

Remember though that hormones are churning around inside of her. Don't take offense at her behavior, but make her part of the solution instead of part of the problem. Ask for her advice about clothes ("What do you think would work with this skirt?") or baking ("Do you think the cupcakes will be good with chocolate frosting or vanilla? and what about sprinkles?"). Read Twilight or the sequels together. Take her to the library. If they have a volunteer program, check into it for both of you. Relax and enjoy her company.

Your being respectful of her will show her how to be respectful of you.

She's 12 years old, and it's 'natural' for pre-teens and teens to rebel. I remember when I started wanting to do everything that my parents didn't want me to do, and I did it, just to get on their nerves. They would offer to do things for me, and I would refuse, but when I wanted a car ride to my friends, they better be dang well ready to clear their schedule!
It's something that most of us go through.

You cannot get respect and be angry, forcefull or controlling, or you are creating an unhealthy "respect." If you think can only gain respect through fear and anger, you are being fooled. These emotions will create an outward "respectfull attitude" and an inward rebellion.
Respect isn't making her obey, putting her on a tight reign, and getting into shouting matches in order to be "superior." Respect her as a person- give her boundaries, but allow her options to make her own decisions. My answer to your question is essentially simple, and comes down to this: to earn respect, you have to show respect.