Dealing with the difficult situation is difficult - usually because emotions are running high and emtoional responses tend to send all good planning straight out the window. One technique I've found useful is simple enough to remember when the flak hits the fan and tends to work well - the DED technique.
D = Describe
E = Example
D - Discussion
Describe the behaviour you're finding difficult, how it makes you feel and explain the wider impacts of the behaviour. Without any description the other person probably has no idea why you are upset.
Then provide an example of what it is you've just described. Without an example the other person may think you're making it up and may not be able to ground the descriptions in reality.
Discussion - or I prefer the term dialogue as that presumes it is a two-way thing - is the obvious important and final step. If the first D and the E have gone well then the discussion should be fairly straightforward - though posisbly still emotive.
Do you have a favourite technique you'd like to share?