Use of Же

[mangiami.livejournal.com]

How do you use же? Is it just placed after whatever you want to emphasize? Are there any times when it should be especially used, like after a positive command using an imperfective verb, such as in:
- Открой окно, пожалуйста.
(after waiting a minute)
Ну, открывай же!

(If this is already answered elsewhere, please refer me to it, and I apologize for this post in that case.)