Bibtex author/date order

Hi List;

I have a situation where I have:

Ward et al. (1999a) referring to earlier work, Ward et al. (1999b)

It looks as if the bibliography is alphabetically ordered based on the
2nd, 3rd and 4th authors. In the case above, because the 3rd and 4th
authors are different this then places these two papers in reversed
chronological order.

Is there a method or "trick" that can be used to swap things in the
bibliography so that:

Ward et al. (1999b) can indeed refer to their earlier work, Ward et al.
(1999a)


Kind Regards
 
Rob S