I think the single biggest thing which can be done is to immediately talk to key people and share vision, strategy and exciting goals and make that key personnel comfortable about his/her all the worries and uncertainties.
Transparency and communication with key employees is valuable to ensure retention. Especially about the future role and development opportunities on personal level, as well as the future growth of the consolidated business.
The target location (locally or crossborder) and size will heavily affect the approach of the acquirer and the stress on the target. Key employees need to be identified as soon as possible in the DD process and clear communication on the strategic goals could reassure these key employees who will otherwise be among the first to leave. Often the stress is related to their future and potential evolution (e.g. new organization structure, role, salary/package, etc.). The idea is to retain the good and key employees while at the same removing inefficient or disruptive ones, as usually, the fact of not taking difficult decisions at the beginning leads to a disruptive employee long-term cost being much higher than the short-term and more difficult decision of addressing the problem early.
Put in retention bonuses that are hard to walk away from. Also, consider elevating key resources positions in the new company. If they lack required competencies, put them on a learning program and assign a senior mentor. This is often the case in acquired IT scenarios, and good IT resources, such as developers are scarce