Use color and graphics that are eye-catching and exhibit a design that matches the product. Heavy Metal or Death Metal advisory stickers can be fun and have skulls and scythes as well as legitimate warnings. Think of the "Jackass" logo and warning, which is like an old-fashioned poison sign with crutches for the X underneath the skull.
Use humor. Use the typographic symbols like @, $, and % to denote foul language. A sticker can be funny and clear about how explicit a CD lyric is if the sticker says, for example "This CD sings about language, ##$$%^, #$%, #$% and drugs." A parent with a sense of humor will understand just how strong the lyrics are.
Consider making the labels collectible. If you are designing a set of parental stickers for several CDs from a band, put one of the band members on each CD as it comes out on the market. For comic books, use the characters. The collectibles can become another sales tool.