How do you help people stay on topic during a workshop?

My two cents

Here is a mix of technics I use to help people stay on topic during a workshop and to help them to not get lost in the details:

Use the wall: capture where the conversation is going with sticky notes. As people change topics, it becomes very visible that suddenly you need to create a new category for the sticky notes, which shows everybody that you are changing the topic.
Use parking: at the start of the workshops, I usually explain that we want to avoid speaking about interesting but unimportant details (see bikeshedding) and that, therefore, we create parking where we capture all things that are outside of the goal of the workshop but that we should not forget about. The parking gets then sent to everyone so that no idea gets lost.
Use your body language: one thing that I sometimes do is to use the physical distance I have with people to signal that they have talked enough. I usually keep a good distance so that I'm not the centre of attention but the person talking. Once the person talks a lot and doesn't want to stop, I slowly come nearer and nearer. This makes the person realise I'm coming into her space and that now I'll interact with her soon to tell her to move on. It's pretty funny how well that works.
Use a timer: giving specific amounts of time for a conversation, and reminding people that they have used half of the time, then reminding them that they have 3 minutes left, then 1 minute left helps to people to be more focused.