The following advice comes from a discussion among a group of History tutors:

1. Purposes
   The key here is to think clearly about what you are trying to achieve; what you want your students to have learned from the seminar. Assuming you want to develop understanding you will need to think about what they already know; their level of experience; degree of interest and difficulty of the topic; the ways they learn new material and such practical issues as the length of the seminar.
2. Preparation
   It's worth remembering that students' confidence in discussion will vary according to their experience. You might want to provide them with a brief agenda with a question or two they might think about and/or allocate specific reading to ensure the topic is kept in focus. Make sure the books you recommend are available.
3. Starting Off
   Students will talk more freely if they feel safe. In a course of seminars, establishing ground rules with students can help to do this, clarifying what's expected as well as motivating the group and allowing them to get to know each other and you. Remembering names is important: name badges can help in the first couple of seminars, as can pairing students and asking them to introduce each other, asking students to state their name at the beginning and end of the first seminar or drawing up a seating list on a flip chart.
4. Completing
   Finishing off a seminar in a positive way is important. Summarising the main points raised can help, and leading into the next topic reinforces direction and provides momentum. Drawing the material together in the final seminar on a course and celebrating the achievement of the group also reinforces its sense of identity and motivation. You might ask students the good things about the seminars and ways they might be improved, or ask them each to list one key thing they learned.
5. Seating
   What works with one set of students might not work with another, but in general how the seating is arranged can markedly influence the atmosphere and make the difference between a relaxed and tense seminar. This might be difficult to change, but an arrangement where you are part of the group rather than separate from it is generally most effective.
6. Methods
   Variety is important here, as it sustains interest and allows a group of students to get to know each other. Buzz groups, brainstorming and pyramiding are classic means of encouraging participation. Giving students thinking time before discussing difficult issues is also a useful technique.
7. Managing group work
   This involves many skills of listening, questioning, responding and explaining. Crucial to these is leadership. Here important things to remember are flexibility and focus. It is important to keep to the focus of the topic, but if the group wants to discuss something relevant but which you have not prepared try to fit it in and miss something else out. Sparking off ideas is usually appreciated but dominating the discussion is not. Try hard not to fall back onto the position of expert, authority and prime talker. It's useful here to think about the research on what students don't like. Pre-eminent here are being made to contribute when you don't want to; being interrogated; being asked vague questions; having your view summarily shot down; tutors appearing bored. Conversely, being genuinely enthusiastic in the topic is regarded as a real plus.
8. Confidence
   Only do what's comfortable. Don't try to force things, especially at first. The important thing is to reflect on what you do, and this is the best basis for experimentation and change.
9. Be pragmatic
   There is no such thing as the perfect seminar, and what works with one group may not work with the next. Some groups will not work effectively no matter what you try to do. There is nothing wrong in admitting defeat with one group: it happens to all tutors at some time.
10. Remember you were a student once
    This is a piece of advice from students themselves and suggests the importance of supporting students and not forgetting the pressures of student life and the transitions they are experiencing.

References:

The Subject Centre for History, Classics & Archaeology
Higher Education Academy