If you need to put in a shower/tub as well as a stool and lav, then follow Tom's excellent advice. But, if all you need is a stoll and lav, there is another option.
I was watching a home improvement show a few weeks ago and they were talking about installing a toilet that was below the sewer lines.
It was a macerating and ejection toilet.
That is a nice way of saying that it takes what you give it, chews it up even more and the spits it out.
Here is a link for it:
http://www.saniflo.com/products/saniplus.asp
Tom (speedball1) and others may have had some experience with this system.