Running retrospectives can be hard work. The outcome can be affected by the dynamics of the group, how controversial the topic of the retro is, and work pressures external to the session. Here are some rules that I’ve found useful when planning and running retrospective sessions.
As Technical Project Managers (TPMs), we’re expected to switch from the manager role to the facilitator role. As
Akash Bhalla writes on his blog:
‘Be invisible… Once you get past the brainstorming, and the team has decided what topics it wants to attempt to discuss and address, this is the point at which to disappear…Now is the time to shift the focus onto the team itself, ask them to form a closed circle with you on the outside and to face each other and talk to each other. As the facilitator you have no business being so heavily involved in this stage.’ 
Feel the pain
There are times when we want to contribute to the discussion, and to do so we need to jump out of the role of facilitator. Before we do so, we need to be reflective; are we trying to fix things? As Esther Derby and Diana Larsen say:
‘…it’s easy to focus on comforting one person and lose track of the goal and the needs of the team.’ 
So, learn to manage your own emotional state:
‘when emotions are high, your team needs someone to stay outside the turmoil. That someone is you, the retrospective leader. Remember, you didn’t cause the emotions in the room, and you don’t have responsibility to make everything and everyone happy and nice.’ 
If you really need to contribute, then organise another facilitator from another team, or rotate the facilitation role around the team (in the spirit of the cross-functional team :-)).
Enjoy the silence
We’re monitoring time, we want some output from the session, and we want people to contribute. So, when the answers are not forthcoming, it can feel uncomfortable not to fill the silence. To quote Esther Derby again:
‘people — especially introverted people — need time to collect their thoughts before they speak. Count to eight s-l-o-w-l-y. If no one has answered, count to eight again. If there’s still no answer, move on.’ 
Have a plan ‘B’
We’re all learning and trying out new things, so it is okay if things don’t go to plan every time.
‘If you get lost in an activity, or something happens that you don’t know quite how to handle, pause and set. No-one can be perfect, so get good at recovering gracefully.’ 
Always have a back-up activity planned just in case, but remember: get the group to decide if it’s working — the session is everyone’s responsibility.
Fail to prepare; prepare to fail
We’re all busy, so preparation can feel like a luxury. We can wing it, using a tried-and-tested formula, and hope for the best. To run and guide a valuable retrospective, we need time for reflection. Akash Bhalla again:
‘…to effectively communicate all the lessons from the past posts takes a planned and structured retrospective, which can only come after some investment into preparation.’ 
Planning might take up to an hour — the same length as the session itself. However:
‘…for the 10 people attending that retrospective, 1 hour of your time taken for preparation will ensure that the most effective use is made for 10 hours of the team’s time’. 
Good Luck :-)