Start by holding up a relatively crisp bill (real, paper money).
Ask how much this bill is worth.
Then crumble it up into a ball, stomp on it, punch it down, do whatever you want to make it yucky.
Finally, unfold the bill and hold it up again. Ask how much it is worth now. Is there anyone who wouldn't want it now that is has been tarnished?
Point out that this is similar to our worth in Heavenly Father's eyes. Nothing we do can detract from our worth to Him.