First I want to say that this is a forum for advice, not judgment, so ignore people's comments that are judgmental. I was spanked as a kid, and I have a great relationship with my parents. I respect them, and while I prefer to use other methods of punishment whenever possible, there are times I've resorted to spanking as well. I, too, have an incredibly smart, independent toddler. She started her terrible two's around 18 mos, and, at 27 mos, we're still in the midst of it. I imagine when baby sister is born next week, that will start a whole new level of it! :) She, too, knows how to push my buttons, and how to manipulate me (I'm embarrassed to say how many times I've been duped!). The problem for me now is that sometimes spanking doesn't even work. (Which I know is what all the anti-spankers will say.) She has learned that I can't really make her do anything, and she's strong-willed enough that no amount of praise, removing of privileges, providing choices or physical punishment is enough to get her to change her mind. Most of the time I try to let her experience the natural consequences (if you won't put your jacket on, you don't get to go outside to play). But the problem is when it's not an option - like getting in her car seat when we're leaving in the morning for me to get to work on time. To my chagrin, I have physically held her down and buckled her in, kicking and screaming. This does nothing except teach her that as long as I'm bigger, I can make her do what I want. But she's getting so strong, and I'm 9 months pregnant, so that's not going to last. But what else to do in those times?
The book "Parenting With Love And Logic" by Foster Cline and Jim Fay has some great advice in it, but I find that some of the information is hard to translate into effective results for toddlers. Lots of it does work though, and I think there's a toddler version of the book, too. I'm re-reading it right now. I would recommend it for you. There's also a book called "The Strong-Willed Child" by Dr. James Dobson that I've heard is an excellent resource. I've been intending to get it, but haven't gotten around to it yet. I'm actually planning to get it this week if I can.
All that to say that I don't have any good answers for you, but I think you are doing the best you can. Hang in there and good luck! This, too, shall pass!