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.
It's a great idea to offer different sized sponsorship packages so a company on a tight budget can still say "Yes" to helping your charity with a smaller donation or purchase of a small sized sponsorship package as well as offering larger packages. This is another great way get more companies to sponsor your charity. Many smaller donors would have to simply say they cannot help you at this time if you only offer one type of sponsorship. If you have banners, offer to sell some space on your banner to some of your larger sized donors. You'll be amazed at how many sponsors will buy ad space.
In the end, just make sure that your donors feel like they are getting something for their sponsorship besides the great feeling of helping a charity. It doesn't take a lot of effort to mention your donors at a rally or include them on a flyer and be sure to tell them that. Every little bit of advertising helps your donors and also looks good for your charity. When people see one company the recognize is involved, it gives a sense of trust and will help expand your efforts at fund raising. So, it's a win-win for both you and your corporate sponsor.