There's never a better time to get started planning than now. No matter how healthy the economy is, some businesses will do great and others will fail. Starting any business takes months of planning and work, so get your business plan in order. While writing the plan you will see very quickly how ready the market is for your business.
Put simply, a business plan is more about the thinking and evaluation you do to be able to write it than it is what you put into the plan. I personally started several businesses during down economic times and found that it made it really to keep costs down. Then when the markets picked up, I was able to make great profits.
All the Best,