S&W M&P Trigger Job
This will give you an understanding of the inner workings of the trigger and all areas which may be addressed, depending on your skill/ comfort level.
Having done a few M&P trigger jobs, just polishing the trigger bar won't net you nearly the improvement of re-profiling/ polishing the striker block. You don't want to remove any material from the trigger bar other than sharp burs. Any burs can be easily removed with a fine stone or very fine sand paper like you use to wet sand a clear coat car finish.
I'm in support of the Flitz and dry fire method stated above if you have no experience in gunsmithing or with a rotary tool. A Dremel is a fast and easy way to screw something up. If you insist on using a Dremel, the grit impregnated rubber polishing tip would be preferable, offering more control and less stock removal.
Hope this helps.