Myosmith

I started riding at 270+ and can tell you first hand that much of the above advice was true for me. I started with a bike too small for me and rode with the seat lower than it should have been (the notorious "barstool" position). I also started with a "comfort" saddle that was the worst investment I ever made. Forgive me if this is blunt, but it was the best piece of advice I got early on: If your thighs already rub together, the last thing you need is a big chunk of foam rubber or gel wedged inbetween them when you are trying to pedal. A seat with a narrow front and properly spaced support for the sitbones (the bottom of your pelvis) is the way to go. Some seats have a cutout that relieves pressure on the tailbone.

Riding shorts with a chamios are a good investment, not for the padding, but for the reduced friction between your upper thighs. You can get styles that are intended to be worn under other clothing if you don't like the spandex look. I wear them under athletic, outdoor sports, or even jean shorts. I also have a couple pair of riding shorts designed for mountainbike riders.

Make sure your bike really fits you correctly. This will enhance your comfort far more than any padded seat could ever dream of. There are some online resources that show you how to fit a bike, but beware that not all of them are accurate. Several books explain the process well. One of my favorites is The Big Book of Bicycling by Emily Furia and the editors of Bicycling Magazine. It devotes an entire chapter to achieving proper fit.

Raising the seat to the proper height will help you tilt your pelvis forward into the proper position without your abdomen and your thighs fighting for space on the upstroke of each turn of the pedals. I found that having my handlebar too low caused back pain when I was heavier, but as I lost weight, I slowly kept lowering it. My bar position 60 pounds ago was about 2" above the level of the seat, but now it's about 1.5 inches below seat level (flat bar on road/touring hybrid) and I can ride comfortably even lower. You should chose a bar height fairly low but which allows you to keep your spine in a natural, straight position to reach the bar comfortably without locking your elbows. If you are rounding your spine to reach the bar, it is too low for your current fitness level. If you are using a flat bar, you can raise the stem or replace it with a better fitting one, or switch to a riser bar which can move the grips in an arc giving you some control of not only height, but of setback as well.

I think it is also important to mention that exercises that strengthen your core muscles (back and abdomen) as well as increasing your flexibility will go a long way to alleviate back pain. Check with your physician if you think your back pain is from an injury or degenerative process (like arthritis) or before starting a new exercise regimen.