Times have changed, and indentations for paragraphs are usually not used because it is easier not to use them. The body paragraphs should be single spaced in a business letter. But you should double space between paragraphs when your letter contains more than one paragraph.
In a second paragraph, you will want to give a specific example of how you benefited from your contact with this person. Be sure to thank him/her for his/her time and efforts on your behalf.
