Mr_Vodka's suggestion is exactly how I would do this.
Here's an example:
Let's say I want entered text to always be bold face...
Find the field's definition in Manage | Database | Fields and double click it.
Select the Auto enter tab Click the Calculated Value CheckBox Enter this expression: TextStyleAdd ( Self ; Bold )
After clicking OK to dismiss the specify calculation dialog, clear the "Do not replace existing value..." check box.
For more text formatting options and functions, look up Text Formatting Functions in the help system.