InDesign's nested style feature is very powerful, but not too many people I've spoke with have really put it to use. It can save you hours of wasted time formatting. For example, consider a glossary style entry:

word (n.) definition goes here

The first step would be to create a character style sheet for the bold text, and another for the italic text. Specify ONLY the attributes that should be specific to this style. This doesn't just apply to nested styles either. It's always applicable. If you set the style to use Helvetica Bold but do not give it a specific point size you'll be able to use that style on any text and it will automatically match the point size of the text around it. I've got a file that uses both Univers and Univers Condensed at 8, 10, and 11 pt in footnotes, sidebars and body text. I only need 2 bold stylesheets. One for Univers Bold and one for Univers Bold Condensed....got it? cool.

So you've got your bold and italic stylesheets. Now make a paragraph stylesheet that will represent the definition portion of the text. Pick a basic font and a size, leading, space before and after, etc.

Now we'll make the nested part of that paragraph style. In the Drop Caps and Nested Styles section of the paragraph stylesheet settings click New Nested Style. You need to choose a character style and specify how long to use it. You can use a style "up to" or "through" any number of words, character, space, or special character. Note: the default is "Words". There are other options in a pull down menu but you can also type anything you want in that box! It could be a colon or an em space...whatever!

For my example, the nested styles look like this:
Bold  through  1  Words
Ital    through  1  Words

After that it reverts back to the original paragraph style specs. (Hint: use a non breaking space if you want multiple words to be styled as 1 word.)