Treat the observer as a "critical friend", who will be using their experience for your benefit, and take control of the situation.
The most useful observations have a focus that you've agreed in advance with your observer. Negotiate the focus, and highlight what you're good at.
It's also a good idea to ask for help with areas where you're not so confident - wanting to get things right is a strength, not a weakness, and most people are flattered when you cast them as the expert.
Remember that in the classroom you're a role-player. That gives you control of what you do. The observer is much more interested in what you do than in making judgements about who you are, so tailor the role to your own advantage.
Everything that happens in your lesson should combine to make it clear to the pupils, and your observer, that learning is taking place. Use your lesson plan to emphasise that promoting learning is your first concern, and hammer that message home.
Begin your lesson plan by asking yourself, "What will pupils learn during this lesson?" The answer will give you your learning objectives, and if they are clearly stated and appropriate, the rest will follow. Write them on your whiteboard, in pupil-friendly language. (If your handwriting is unimpressive, use a data projector for clarity and impact).
Deal with teaching objectives and learning outcomes in this part of the plan, too. Teaching objectives should explain what you are going to do, and how your activity relates to national curriculum or framework objectives.
Learning outcomes describe what pupils will be able to do with the new knowledge they have gained as a result of their learning. Express them in the "all pupilsmost pupils some pupils will be able to", and thereby you also show that you're aware of the need to differentiate.
Plan your starter carefully, and keep it short and sharp. It helps if it in some way links directly to the main body of the lesson, perhaps by introducing skills that pupils will need to use for that. What's the point of a starter that exists in a vacuum?
Make the body of your lesson as clear and logical as possible - write this part of your plan so that your observer can see each step in a clear sequence of activities. Build in opportunities to make pupils take responsibility for their learning, by introducing, as appropriate, pair and group work, reporting back, peer and self assessment, discussion of success criteria. Let pupils use your equipment, such as the interactive whiteboard.
If you possibly can, let your plan reflect your use of assessment information and individual education plans - make it plain that you are meeting pupils' learning needs.
Use quiet showmanship in your delivery - but think Derren Brown rather than Noel Edmonds. Brown claims to base his "mind control" on neurolinguistic programming, and you can do something similar. You want your pupils to learn, and you want your observer to see evidence of this - so, at every opportunity, use the words "learn" and "learning", and consolidate the message by asking pupils to tell you what learning has taken place.
To give your lesson a polished feel, pay particular attention to transitions - if they go smoothly, everyone will feel that you're competent. The key is to make your instructions as clear as possible, and to have one or two pupils tell you exactly what they understand your instructions to have been.
Finally, ask for a debrief after your lesson - oral feedback promotes the kind of discussion that will help you focus.