sorting bibliography by authors

Hi,

I have bibliography entry which is categorized into Article, Books, Web 
Documents etc. etc. Each has many random entries. I am using 
customization layer to sort the authors name in surname,firstname order. 
Besides that I want the entries in each categoy to be sorted ascending 
by surname and year.

For example,
------------

Engel, L. (2004)......
...
...
Shrestha,D.(1999).........
Shrestha,D. (2000)......
-----------------------

How can I accomplish that? What thing I need to add in my customization 
layer to do this?

thanks in advance

Deepak K. Shrestha