With seminars, the danger in any length over about 10 minutes is getting lost in a morass of detail, and letting go the thread of argument - so good organisation is essential. Stick strictly to the topic you have chosen. It goes without saying that your papers will be the result of deep thought and extended but well-focussed reading. If you are using slides, make sure you know how the projector works. Practice beforehand, perhaps using one of the viewers in Room 113 or the Slide Room.
Nerves affect different people in different ways.
If you are nervous, then preliminary planning is all the more important, otherwise any hiccup may throw you. Few people are able to be genuinely witty about works of art, and over-egging the pudding is a constant danger. Depending on whether or not you are nervous, you will want to consider differing ways of presenting your work. Alternatively, you may want to speak from notes. Doing so may well sound much more natural than reading it out, but it is a skill to be acquired, and a useful one.