Screenshots

Introduction

Sample screenshot from SAP R/3

Always capture screenshots full-screen, unless you are describing only a specific section of the screen. In this case, you should use some illustrative convention to indicate that the screen shot is a partial screenshot and not of the whole screen.
Always capture screenshots at a consistent scale. This is especially true for pop-ups and dialog boxes, which should always be shown at the same scale as full-screen screenshots in the same publication. It is acceptable to show specific elements of the screen (such as buttons and toolbar icons) at a larger scale (than the full-screen screenshots) as long as they are cropped appropriately (that is, to show only the element itself, and without any extraneous portions of the screen around the outside).
Always capture screenshots the way that the user will see them - that is, using the standard color palette, and with no user customization (unless this is what you are describing).
Avoid 'junk' in the screenshots - such as additional programs in the Windows Task bar.
Beware of what information is shown in the screenshots - do not include confidential or sensitive information. But do try to use 'realistic' data (not "Dirk's Dummy Company", or "TEST123") and show realistic scenarios. You may want to consider including a disclaimer such as: "Screen prints show sample data to be used for training purposes only.", or something similar.