I mostly do ghostwriting and co-writing projects these days, and each deal has been a little different.
The first time, it was a straight 50/50 split of everything (advance and royalties).
Another didn't garner enough of an advance to make it worth it for me to do a split, so I basically got the entire advance, with the subject earning the bulk of the royalties.
With many of my projects, though, I'm brought in after the deal has already been made, and I never know exactly how much the deal was between the publisher and the celebrity. I'm offered a flat rate (plus a portion of royalties, usually) by either the publisher or the celeb's agent and I can decide if it's worth it for me regardless of what the celeb is getting. That is, if someone is offering me $50 grand plus expenses and a cut of the royalties, I don't really care how much the celeb is making from the deal. I would probably consider it a fair deal for me.
I assume that you're in it from the proposal stage, though?
I'd negotiate for a flat fee for the proposal itself. The first portion of the advance payment almost always goes straight to the writer because you're the one who needs money while you work; the latter portions of the advance may be split, or, depending on how the advance is paid out, the writer may be paid completely first, then the celeb gets paid.
Do you have significant magazine credits, or other types of writing credits? Even if you have no book credits, that can help.
P.S. I'm not clear on whether you mean you're already in negotiations with a publisher, or with the celebrity, but if the former... for future reference, ALWAYS make sure you have a signed contract with the celebrity first that outlines your deal before you begin contacting publishers. You don't want to find out later that the celeb expects to pay you 10% and that you've done all this work for nothing.