Saleh Abdo, Oman
The observer should have a clear idea of what is he going to observe. Is he or she going to observe: the suitability of the material to that particular group of learning? the students' progress? What aspect of the lesson? The teaching techniques, the teachers presentation, the students' practice, the feedback or the teachers' role? He /She should have a definite aim and the persons observed should be informed of the observation aim.
If the observer and the teacher observed have a definite idea and a clear aim, they will make the visit fruitful.

Gill Wilson, England
Regarding observations, you could discuss with the teacher before the class what his/her aims for the lesson are, and what the outline plan for the lesson is. You can then make an objective assessment of how you feel the lesson measured up to the aims or plan. You can simply record the lesson by noting what the teacher did, what activities the students did, whether they worked in pairs/groups/alone etc. This will give you an idea of the way the teacher balanced the lesson. Make notes as well on the atmosphere in the class, the way the students interacted with the teacher and each other and any other points that seemed important in the lesson.Good luck!

With regard to being observed myself, I see it as an opportunity to learn something. An observer can often see things that we don't notice ourselves.

Joe
At the University where I teach we have the regular practice of observing each others' lessons - teachers and instructors are to attend two or three of their colleagues' lessons each term. Besides this, we have so-called "open" lessons, where a whole board of teachers is present.

When being observed, at first I feel uncomfortable, but once the lesson is in gear, I concentrate on my students and the lesson and stop worrying about who is present and how many of them. And I say to myself "They, who are sitting now in the back row, are also students, they've come to learn, not to judge you".

"Open" lessons certainly need more thorough preparation than usual ones. First of all I choose which group of students could "perform" it - they should be active and mobile and not absolute beginners. I warn the group beforehand and tell them it is not they, who will be observed, but I and my work. Then I choose some lively topic which may be of interest not only to the students but to observers too! Sometimes I try to involve the teachers present in the activities (a few agree and are lucky to see how the lesson works).The lesson plan itself should be well balanced, interesting, have some bits of new technologies or some unusual approach.
I always try to keep the tempo, but at an "open" lesson it is of special importance, or it will seem boring to the observer. One or two days before the lesson I start the topic with the group - we work on the vocabulary, do some grammar bits, read and listen: they get familiar with the topic and don't feel lost at the open lesson. But! I never "rehearse" the planned lesson with the students - this is what some teachers do, but the result is always pitiful and one can easily tell when the lesson is a "staged" one.

After such lesson our Chair usually holds a meeting and discuss its good
points and faults. The points are:-
What were the goals of the lesson and were they achieved?
What objectives did the teacher put and were they completed?
How well was the lesson planned?
Did the activities used match the goal, the students' level, etc.?
What new things could we learn from the lesson as teachers? As students?
Was the lesson successful?
Was there anything that could have been done in other ways?

While sharing we pay attention to how the teacher dealt with the students, what was said and how, the tone, the atmosphere, the students' reactions. etc. Such lessons are challenging and take much effort, but it gives an opportunity to learn to both the observers and those being observed. The teachers get a lot of advice and can view their lesson from the others' points of view. And we discuss not the teacher, but their lesson! Only having attended several lessons of a teacher can one judge their professional level.

And one piece of advice to dear colleagues: do not fear being observed! If the person to sit in at your lesson is a professional - you will learn something new and useful about your work, if they are not - why worry? They will understand nothing about your work, just don't forget to smile and prepare well for the lesson :)

El fallito, Spain
Teachers have to be prepared to be observed. The obvious problem is arousing suspicion about your abilities by refusing to be observed, but it can also lead to helpful criticism from a third party, i.e. a non-teacher in this case. Just make sure you prepare the lesson the best you can, and have your answers ready at the end!!