Choosing a textbook can be a difficult task, especially as there are so many on the market and publishing reps might be visiting you to persuade you to take their books. It's also a very important decision as when you choose a book, you and your teachers are commited to it for some time. For this reason, I think it's best to involve the teachers as much as possible.
If you can, have a meeting with the teachers who'll be using the book. Have a selection of coursebooks available and try to come to some sort of joint decision. Even if you can't all agree, at least teachers feel involved and will understand the difficulties involved. This should reduce the amount of complaining they indulge in afterwards.
Have a list of areas to consider and ask teachers if they want to include any others. Teachers can then split into groups and look at the books, referring to the checklist. Have a feedback session at the end where everybody gives their opinion and a choice is made.
Here are my suggestions for areas to look at:
1 Syllabus. Is it suited to the aims of your course? Is it pitched at the right level? Intermediate level books, for example, can vary in level. Just as size 12 clothes do!
2 Topics: are the subjects likely to interest your students?
3 Depth: do the books give enough practice in each language item or will they need to be supplemented?
4 Skills: how are the four skills dealt with? does this match the aims for your course?
5 Activities: how is vocab dealt with? grammar? pronunciation? Does this fit in with your needs?
6 Supplementary materials: is there a teacher's book? a workbook? audio cassette? video? What are they like and are they easily available?
7 Availability: the best book in the world won't work if you can't get it delivered to your school on time!
8 Flickability: this refers to the attractiveness of the coursebook. Does it look appealing and interesting?