While it's common to go into an "edit mode" to adjust data, edit-in-place allows customers to perform quick, simple edits simply by clicking the text to be edited. By keeping the customer in the same context and letting their mouse clicks switch between "view mode" and "edit mode" at will, they'll get things done quickly and get less frustrated about wasted time. Flickr.com is well-known for its edit-in-place titles and descriptions for any photo you've uploaded. A good best practice to follow is a hover state affordance: the editable area highlights in pale yellow and a "click to edit" tooltip appears.

 
If you run an e-commerce business, think about using this technique in a shopping cart -- the prices instantly update when you change a quantity or the shipping ZIP code.