Set your goals in concrete but place your plans in sand. I don't know who originally said that. It's been quoted a lot, without attribution, and it is one of the best things I have heard. Think long and hard about your goals and, when you finally decide upon them, keep them out there in front of you, solid, unchanging, as if built on a foundation of bedrock. Create a plan to reach those goals but remember, if your run into trouble, if situations change, you can always change your plan, or find another one entirely, in order to reach those goals.
Evaluate your business plan regularly. Don't be afraid to show it to others. Let people review it. Get their opinions. Listen to what they say. Evaluate their comments against what you expect to achieve. Then make any appropriate changes that need to be made in your plan that will help move you along to your goals. Don't be afraid to make changes but also don't run willy-nilly in every dirction just because people offer a variety of ideas and advice. Make change where it is needed; stick to your guns where it is not.
There are lots of people you can turn to who may be willing to review, and comment upon, your business plan. Some that come quickly to mind include members of your family. They will almost always offer comment -- often telling you why it won't work. Don't be discouraged. Sometimes there are grains of truth in what they offer.
Any investors in your business should see your business plan anyway. Ask them to comment.
Your banker is a good choice. It won't cost you anything and he has a vested interest in your success. Remember, though, a banker is not a business person. He will typically tend to err on the side of conservatism.
Your lawyer may charge a fee to review your plan but his advice may be invaluable too.
Business mentors and trusted business friends who have been in the trenches are a good choice. Potential customers, and even competitors (distant ones of course) are possibles, too.
The U.S. Small Business Administration has an online Small Business Planner that provides a lot of information and should help develop your ideas.