The economy isn't the best and although it may not seem like a very opportune time for asking for a business to sponsor your charity, in all actuality, the timing couldn't be better.
Remember, a business may give your charity funds to meet the needs of whatever your charitable cause is, but you give something back to the business for their funding. They are getting extremely low cost exposure (advertising) and you need to promote that fact to them. If you're having a fund raiser, be sure to tell your potential donors that you will be promoting their business as well.