In my view, it's best if you are seated. When you lie down, your body goes into automatic sleep mode, and you end up going unconscious. When you are wanting to meditate, and end up sleeping instead, you really are not getting the benefit of the meditation. The idea with meditation is to be relaxed, but still aware and awake.
What makes you feel uncomfortable when you're sitting? What type of chair are you sitting in, and are you having the same experience in different chairs?
I echo what everyone else says, that sitting is better than lying down. I have gotten into the practice of positioning my back flush against the wall so that I'm supported. The more I practiced, the less uncomfortable I found myself. Slipping into a blissful state is so very easy for me now when meditating, so if I was uncomfortable before meditating, I'm certainly not uncomfortable during meditation!
You might just have to stick with it for a few weeks. It does get easier.
I think sitting on the floor in the lotus position is the most conducive to great meditations, but that's just not very comfortable to me anymore. It started getting less "fun" to sit that way when I hit about 50 - but that's limited thinking and I probably should get my groove back on that, so to speak. Sitting lotus with your back against the wall helps, or sitting in semi-lotus is an alternative if lotus is difficult for you.
Otherwise, sit in a chair because you'll probably fall asleep or not focus, as others said.
I'll move my PS down here since you're talking about the same thing. We must be vibing that together. :-)
Also, whatever position you choose, keep your back straight and your chakras lined up so energy flows easily. :-)
I don't think lying on your side would be a good idea because your spine would be crooked and your chakras would not be lined up. If you do use a wall for support, you're right, you can't totally just lean on it. You still have to keep your spine straight, good posture, etc. I've never felt pain lying down, but I am among those who fall asleep. I'd say sit up straight and use a chair or wall for support HELP if your back pain is an issue.