A business plan is a good tool to help you focus your business and define your goals. It isn't hard to write a business plan. Even if you don't write out a formal document, even if no one but you will ever see it, it's a good idea to at least sketch these things out for yourself. It can be as long or short as you want, as detailed as you want. If you ever need financing to expand your business, it's a good thing to have on hand. Banks will know you're serious if you have a business plan already written.
Here's what to write in your business plan:
a short (1 page max) history of your business
a description of what your business currently does, preferably using figures: annual sales, breakdown by category, etc. "Last year we sold $14,000 worth of baby booties, attended 3 craft shows, added 100 new customers, acquired a color laser printer," etc.
where your business fits in in your industry: retail and/or wholesale? what's your approach to business? what need in the marketplace does your business satisfy? who's your competition?
who your target market is, and how you plan to reach them this year
(optional) any community outreach you may be planning: fundraisers for your favorite causes, targets for community giving, and how these affect your business
your business goals for this year, including measures of success-- e.g. sales volume goals, goals for numbers of new customers, profitability goals, etc.-- so that you'll be able to know when you've accomplished your goals. "Increase sales" isn't as good as "Increase sales by 10%". There's no penalty for not meeting your goal, but the goal helps you get focused on a target and (if you share your business plan) communicates to others what your ambitions are, whether they're realistic, etc.
If you don't have numbers to attach to your goals (like sales volume for example) then set goals that don't have numbers, like "attend a large craft show". You can change it mid-year too. I added "work on making my craft show booth more attractive" to 2010's business plan. The point is to put in writing something that says where you've been, where you are, and where you're going. By putting it in writing, you can give form to your thoughts and create a record by which to evaluate your business' progress this time next year.
What are you putting in YOUR business plan for 2011? Are you writing a formal report, or sketching it out on a napkin?