What to look for when mattress shopping?

I am in the process of re-decorating my bedroom and I am currently matress shopping. I own a waterbed so I've never done any mattress shopping before. I have no idea where to begin. Matresses seem so expensive and there are so many to choose from. Where do I start? What should I be looking for? What are some inexpensive brands or inexpensive places to shop?

foam mattress research links

http://mattress.talkspot.com/aspx/templates/blank.aspx?msgid=0

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