Define your measure of ‘success.’ Is the goal to retain all employees or top performers?
Define the specific audience of who needs to be retained. (i.e. management executives, talented engineers, location-specific employees, etc.) Set criteria based on the priorities of the target audience. Individuals may vary widely so discovering the true care-about issues is the best way to ensure retention.
For execution compensation, time-based or performance-based incentives through cash compensation, equity or variable bonuses may be considered.
For responsibilities and work-related duties for engineers, investment in R&D for projects or flexibility in choosing team members could be desirable options. The goal may be to retain but also to boost motivation and encourage the best performance from people. Specialists might be motivated by freedom to implement their own ideas into product development whereas generalists may appreciate team-building and achieving metrics.
Cultural considerations may impact retention efforts, particularly if the acquirer has a drastically different culture. For example, Europeans may value time off, Americans may want more variable pay than Asians. Whether the culture is based on geographic location or management style (start-up culture vs corporate), a detailed plan specific to your situation will likely produce better results than a generic program.