You can use the following fecth statement for the onchange event of the account look up.
crmForm.all.new_reportedid.lookupbrowse = 1; // This turn on filter crmForm.all.new_reportedid.additionalparams = "fetchXml=
crmForm.all.sa_customerid.DataValue.id = This should be the field name of the account id in your form.
selObjects=2 - referes to the object type code of the entity. Normally contact entity has the object type as 2.
crmForm.all.new_reportedid = This should be the field name of the contact look up.
selObjects variable specifies the object type code of the entity.
This is something that i had to do in one my projects using CRM 3.0. i was greately helped by the artical posted by Ronal Lemman. you can find his article at http://ronaldlemmen.blogspot.com/2006/12/filter-data-in-crm-lookup-field.html