Even though most family historians are not "professional" researchers, presenting a professional appearance and attitude while doing your research will help you get the information you need.
When visiting libraries or courthouses, or when you are out in the community conducting interviews, you will make a much better impression if you dress appropriately. People will be more willing to help you if you don't come into their facility looking like you just changed the oil in your car.
Make sure you have a pleasant attitude and thank those people that have helped you. I had a client who told me that he had been getting the runaround at a particular courthouse and had been unable to obtain certain documents. He hired me to get the documents for him. I politely introduced myself to the courthouse clerk and asked for her assistance. I showed her the list of documents that I was trying to secure. She recognized the names and immediately said that another person had come in approximately a month earlier looking for the same documents. She went on and on about how this man had expected her to drop everything she was doing and help him. I am sure she exaggerated the situation, but that is the impression she got. She viewed him as an amateur not worth her time, and she viewed me as a professional she was willing to assist. I was able to get all of the documents with no problem.
When visiting a genealogical library, familiarize yourself with the rules and abide by them. For example, most genealogical libraries will not allow you to bring in any type of purse or tote bag, nor can you wear a jacket or coat. You may only bring a clipboard or a notebook and a pen. These libraries have books that simply cannot be replaced, and they cannot afford to let someone walk off with them. It might be a small inconvenience, but everyone benefits. Don't make a big deal about it. If there is a time limit on using the microfilm reader, let someone else have it when your time is up.
You can have calling cards printed up very inexpensively. Go for the subdued look and not the bold and flashy (neon and historical research just don't seem to go together). A calling card shows that you conduct your research in a professional manner. You also make it very easy for the recipient to get back in touch with you.
Use quality stationary for your correspondence. You can set up letterhead and envelope templates in your computer word processing program. You are more likely to get a quick response when you send someone a well-written letter on nice paper instead of a hand-scrawled note.
These are some very easy ways to improve the way you do your research. You will increase your credibility and get better results if you conduct your business in a professional manner, even if you are not a "professional" researcher.