Protecting woodwork before you paint (or do other work) around it will save you time and money later. Trim protection tape comes in a variety of widths for any trim detail, from moldings to banisters. Look for a product that's easy to use, will stick solidly to surfaces for as long as your project lasts, and won't leave a sticky residue when it's removed.
For more on trim and molding, consider:
How To: Paint Trim
Quick Tip: Installing Crown Molding
Get Trim! 9 Ways to Dress Up a Room with Molding