One important prerequisite to consistent messaging is determining your position long before the conversations with buyers get under way. This is important for a couple of reasons. Firstly, in most situations there are multiple shareholders and often there are differing viewpoints regarding valuation, structure, future employment goals etc. 
Sitting down behind closed doors to determine your collective position up front ensures that everyone is aligned, enabling you to communicate with one voice, avoiding any mixed signals that buyers could take advantage of or be confused by.
Secondly, and perhaps most importantly, by determining your position up front, you avoid the trap of making on-the-spot decisions in the heat of the moment that could prove suboptimal. Having clarity early on regarding the endgame you are seeking is an important factor to being successful in this process.
Negotiation is an on-going process that begins even before numbers and figures are put on the table; make sure you are clear on your position from the outset and that you maintain a consistent message.