A couple of thoughts about sleep fighting. Just want to preface by saying these thoughts come out of my experience as someone who was a sleep fighter throughout childhood and remembers how she felt, has watched the sleep fight with some family members, and is a proponent of The Continuum Concept -- AND has one kid (just one) who has had uneven sleep habits but no routine. I've been trying to work through my assumptions about sleep and about my role in it. So anything I say is just the framework that is currently working for me. I don't mean to imply anything negative about what you're already doing, or anything at all about your child's temperament. One background assumption I should advertise has to do with my being Catholic: I just take it for granted that everything about my life is a little skewed. The fact that my dd doesn't drift off effortlessly as soon as she starts yawning doesn't mean that the idea of natural sleep habits is wrong. It means I'm haven't fully devoted myself to it or uncovered my inner demons about it. I say that a bit tongue in cheek, but the pattern still annoys the heck out of some people. So I love you all dearly, and am open to being set straight by you. Just sharing my beliefs and experiences. End disclaimer. :-)
First, are you sure this is a fight? Or is your child tired, but something is keeping her from getting to sleep? Try to step back from worrying about sleep and trying to get your child to do it, and think about what is standing in the way.
One thing that I suspect is pretty common and was a huge issue for my dd is energy discharge. Try slinging not with the immediate goal of putting your baby to sleep, but in order to do something very physically active. Go for a brisk walk for fun, or do some wild dancing, or do some demanding, moving-around type house or yard work. Or, when s/he is seeming overtired or maybe after a bath, play wildly: dangle her from each limb and from her head (supporting each side with your hands), hold her upside down by her legs, toss her up in the air. Then stretch and massage her entire body. Then listen to her: is she ready to nurse, bounce, ... ? If energy discharge is the problem, your baby will probably feel a lot better after some of this stuff and go to sleep more easily.
Since we have a lot of electric lights and follow a non-light-oriented schedule, some babies have an internal circadian rhythm that is out of synch with their environment. So sometimes it is helpful to pick a time (whatever seems natural to you, knowing your baby and the natural lighting where you live) after which you keep most of the lights off and the rest low, and try to consciously be quieter and do quiet activities. (So in my case, at one point while I was in grad school it occurred to me that my habit of suddenly deciding to sweep and mop all the floors at 9:30 pm was probably keeping dd awake. Oops.)
Think of you and baby as on the same team, and when baby is tired and can't seem to sleep, you are trying to find out how you can help. You might have to try different things. If you have a non-nursing partner handy, s/he might be better able to help sometimes. For example, my dd went through a period where all that would help her sleep was bouncing up and down on my dh's lap. His, specifically. The important thing -- which can be hard when you're frustrated -- is to avoid thinking that you "know better" and your baby should just cooperate, because no one of any age likes this and it tends to be counterproductive. You may come to an insight into the situation (like the energy discharge, or the lighting, or whatever) that your baby doesn't have, but s/he remains the ultimate authority on her own body (though with you thinking sym-pathetically, in union with her, as much as possible).
And then there is always food or elimination habits. Could s/he be allergic to something, feeling ill or hyper? Needing to have a bowel movement?
Sometimes, for whatever reason, you are not going to be able to help your baby go to sleep. Maybe s/he is miserable from teething (and your frozen washcloths and whatnot haven't worked), or truly doesn't want help and doesn't want sleep -- learning and exploring may be more important right now. (Like when I stay up half the night to read a really good book. The sacrifice is worth it. And if I *really really* need the sleep, I'll collapse later.) At that point, it's ok to just let it go. You don't have to feel like a bad parent because your baby is up late, or worry that her brain isn't going to develop properly, or start reading Marc Weissbluth. If s/he gets upset and overtired later, continue to be calmly supportive. But don't act desperate or make an issue of sleep. When you are ready for bed, take her with you. If she won't settle down, take turns with your partner being the one to stay up (in the dark) with her. If you're sleep deprived, find someone to babysit the next day and take a nap.
I guess the insight that is really important for me to hang onto is that my baby is not broken. S/he may be different, and there may be some hard aspects to that. S/he may be going through something different right now. But there is no way s/he is simply self-destructive. I need to adjust my expectations (to make sure I'm not conveying any negative ones), deal with my own feelings of anxiety and frustration, and continue to be calmly supportive. There is nothing wrong with listening to your baby's cues and responding. If that means that every night at exactly 7:30 your baby acts tired and is happiest if you read a book and then tuck him into his crib, great! Go for it. If that isn't your baby, you don't have to do anything special other than continuing to take care of yourself and be supportive. Usually, unless there is a medical problem, these things will ultimately take care of themselves.