Hi Snoopy,
Not sure where you are located but in the UK make sure you select a pest control service which is BPCA (British Pest Control Association) registered. Some must haves for pest control:-
Regular visits (8 technician visits plus 2 or 4 QA visits per annum are normal)
Detailed specification covering all pests
Follow up procedure (ie re-visits by contractor following a problem, they should re-visit three times or until problem is clear)
Site plan showing location of bait boxes, insect detectors and electric fly killers (EFKs)
EFK tube change in spring (breeding season)
Safety data sheets for all chemicals used in pest control book
Policy on toxic / non-toxic baits (toxic being phased out in UK)
CVs of technicians (some form of professional training or accreditation important)
Proofing advice offered (although it is normally cheaper to do the actual proofing yourself than get them to do it)
In my experience small local firms can be better than national ones; a good technician who understands your business is crucial.
Hope this helps.