Use a soft toothbrush.

Im not sure if it will work in the crevices but an old CD as a scraper will clean nonstick cookware with out scratching.
Other wise I would suggest putting boiling water into it covering with foil until the burnt on comes off.(covering with foil to hold the heat) If that doesnt work I would use the same method and bake it.
soaking with boiling water usually removes anything.

Thank's for the Suggestions !!
Will soak in hot boiling water ! then perhaps if needed
use a soft brush ! That should do it !
Olga,

Before putting the batter into it, I normally coated the surface with Pam spray. It makes it easy to get the cake out and cleaning it. Soaking in warm soapy water too makes it a sinch to clean.