Check carefully before adopting a trademark or packaging style to make sure you're not infringing on someone else's. It's expensive to establish recognition, and you wouldn't want to have to start over.
After a thorough search, consider registering your trademark. It's not required by law and it can cost about $1,000, but registering helps you build a case in the event someone infringes on your mark.
Make your packaging as distinctive as possible. In designing your package, make sure it doesn't create customer confusion about whose is whose. If you're interested in selling a product with packaging similar to one already on the market, check for existing patents.
Police your trademark. In yet another recent case, the makers of a baby carrier lost a trade dress lawsuit because they waited nine months before objecting. You put a lot of money into your trademark and trade dress, so stand up for your rights.