She probably doesn't want to miss any of the fun, so that's why she fights her naps. I've read that if you make the pre-nap time boring and quiet, she'll feel less compelled to stay awake. Keep the t.v. off, casually draw the curtains, talk slower and quieter, etc. Put away the loud noise-making toys and bring out the soft, cuddly, mute-colored ones.
Then, maybe you could read to her or make it a cuddle time, or "nap together" and when she nods off you can tip-toe away. Or, as they say, sleep when the baby sleeps. And the toddler, too! This way, instead of a fight, it becomes a really sweet time together with both of your girls.
I've also found that neurofeedback works wonders. Basically, like watching fish in an aquarium, your brain follows patterns of high activity vs. low activity. Watching slow, round fish movements slows brain activity, the heart rate, etc. and is restful. Neurofeedback in music is really nice for various needs. I've used it to help me concentrate when I went back to school, and I have another kind that helps me to sleep at night now that I'm so late in my pregnancy and sleep has become an issue.
There are two websites that I like, and you can order CDs or mp3s, or even tapes. There's brain-sync.com and hemi-sync.com. It really works! Even my hyper little dog gets sleepy when we play the meditation one.