- Change lookupsingle.aspx page
http://crm.georged.id.au/post/2008/02/16/Filtering-lookup-data-in-CRM-4.aspx
2. Buy stunware filtered lookup tool
http://www.stunnware.com/default.aspx?area=products&group=fld4&subarea=fld4-download
3. Using JS Code
http://advantageworks.blogspot.com/2008/02/pseudo-filtered-lookup-dialog-in.html
4. Writing plugin.