The time before and after a merger is one of immense uncertainty for employees. Not only are they feeling insecure about the future of their jobs but often their entire remuneration package comes under scrutiny. The best way to deal with this uncertainty is to provide them with regular information. Even if you have got nothing to say, tell them what you are working on and when you expect to be able to make an announcement. Employees should also be told about the implications of the Transfer of Undertakings (Protection of Employment) Regulations (Tupe) for their terms and conditions.
Make sure you have partners buy-in to what you are trying to achieve because they will generally be the conduit for getting the message to the wider workforce.
When you have something to tell employees, do it as personally as possible and explain the reasons behind the decisions. Take teams of human resources people to your sites and let employees ask questions. Most importantly, do not try to sweep negative changes under the carpet, it will lead to ill will and suspicion about the new employer. Remember that communication is a two-way process. Listen to the feedback provided through employee groups and line managers.