Given below are a few useful links that provide help on improving website visitors conversions, may it be subscribing to newsletter or a sale.
http://www.addedbytes.com/articles/online-marketing/ten-ways-to-improve-your-website-conversion-rate/
http://sixrevisions.com/usabilityaccessibility/how-to-increase-conversions-on-any-website-in-45-minutes/