Communication is the key, ensure that people working in different departments are aware of who works where and what they do etc..
Meetings, presentations and social occasions coudl help this as could training sessions with relevant literature on who does what where.
Possibly customer databases with search facilities and regular reviews, maybe a regular group with reps from each section who regularly meet for updates and such and report back to their relevant sections.
A regular newsletter.
Triggers for referrals eg in a law firm where someone divorces/buys a property a referral to the wills team. Customers can be made aware of this in correspondence eg closing letters with a leaflet or contact details.
Documenting these kind of things so they can be followed up appropriately.
Customer focus groups to get their opinions.
Hope this helps :)