It would be advisable to put a longer notice period in their contracts of employment in order adequately to prepare for their departure, and the handover required to a replacement. You may also want to reserve the right to place the employee on 'garden leave' during the notice period.
Given their seniority, and access to commercially sensitive confidential information and customer contracts, it would also be sensible to consider whether restraint of trade clauses such as non-solicitation of clients and/or staff and detailed confidentiality clauses should be included within the contract, and whether there should be a restriction on activities 'preparatory to' setting up in competition. Take advice.
There may be agreements on commission or bonus payments which need careful drafting. For directors, you also need to think about provisions relating to their position as directors - it is legally possible to sack a director as an employee but for them to remain in office as a director - ie how you would handle the termination of the office if their employment were to be terminated.
