Bookkeeping Methods

I am interested in adding bookkeeping services to my virtual assistant business. I wanted to get feedback from others who have done the same thing. What method do you use to get clients documents, receipts, etc? What methods has worked for you, if you don't mind sharing. Also I am trying to target a specific industry that may have a higher need than another industry. In your business, can you say that a certain type of industry has given you more clients than another industry?

Thanks for your help!