Make sure you don't make misleading or unreasonable claims on your website, and have a disclaimer that is easy to access.
Read these recommendations from the US FTC for online businesses - DotComGuide
For scary examples, visit these sites:
example1
example2
As of 1 Nov 2003, if you have a mailing list, you need to ensure you have their name, permission, and agreement to accept advertising from you, and possibly third parties.