After all the time you've spent developing your business idea, the last thing you need is for someone to steal your ideas and beat you to market. There are many instances where you'll want to share confidential information with another party. But the key is making sure that the other party will be bound to respect the confidential information you provide them and not use the information to your detriment.
Thankfully, most people you'll show your business plan to are professionals. If they think you have a viable idea, they'll want to invest in it or join your team. And in that case, it's in their best interest to keep your idea confidential. Just to be on the safe side, though, describe your product or service in your business plan, but don't include a full set of blueprints: critical data such as circuit designs or the confidential pricing with key clients.
To limit the circulation of your business plans, clearly state on the cover page of your plan that photocopying or duplicating your business plan is strictly prohibited. You can go one step further and ask that anyone who reads your business plan sign a confidentiality or nondisclosure agreement (NDA).