Chin Compression Strap to Prevent Double Chin?

Overall most double chins are generally due to fat accumulation. Using a compression strap, in my opinion, is highly unlikely to provide you with any benefit in terms of prevention or improvement either short-term or long-term.

The fullness under the chin can be a combination of structrual anatomy and fat deposit.  A chin sling probably will not change this.  Short of some surgical intervention, for example, sliding genioplasty, liposuction, or a combination there is not permanant change.  Good luck to you.