I use an elliptical trainer regularly, but often experience knee and lower back pain. what orthotics do you recommend? i also have flat feet.

See details: Good running shoes are often all you need. If you need more than that, see a podiatrist and have custom orthotics made.

Running store: Some sports stores have machines where u stand on and measures pressure gradients/differences between balls and heels and then suggests what kind of Orthotic. Lynco orthotics are excellent and different support systems. Custom made orthotics work too, but they are stiffer and sometimes don't fit in as many different shoes.