An effective way of getting good PR and marketing for a company is charity sponsorship. Charity sponsorship will allow a business to get its name out there in a positive light. As well, people generally don't feel that buying a banner in sponsorship is really marketing as much as community development.
However, when marketing with community sponsorship it is important to choose causes that are not political in nature for your business. Some safe ones are cancer benefits, environmental causes, protecting animals (but not so far as say PETA), community development projects, and any physical disability programs.
The size of donation can be important too. While contributing is always a good idea, being a title sponsor when you target customer is an average economic class can be tricky. They may start to see your business as making a lot of money off of them and in turn be less willing to spend. However, if you are portrayed as a small business who finds a little room to do a little good, well, you have found the correct mix of giving and marketing.
Last, it is important to research the pros and cons of charity sponsorship for small businesses to make sure that an adequate amount of money is going to the actual organization and not administrative costs. Some charities end up giving less than 5% of revenue to the actual cause and the rest is paid to high salary fundraisers. It is unfortunate that these businesses occur to make money off of the businesses trying to help them, but it is a fact of our capitalistic society. Be sure you choose your charity carefully to make sure you do the most good not just for the charity, but also for yourself. Requesting a copy of the charities business plan is a good start.