There are a number of good sources of information for this, including starting with asking your local extension educator. They may also suggest other sources of information and put you in contact with them, such as experienced people in the field, farmer's cooperatives, and other people that exhibit livestock successfully.
Several of the breed associations have excellent publications on this subject. Here are a couple of websites, more exist:
http://www.maine-anjou.org/fitting.html PDF
http://www.angus.org/pubs/Showing.pdf (PDF)
http://www.nalf.org/programs/nalja/createwinnerfinal2003.pdf (PDF)
http://www.nalf.org/programs/nalja/feedwinnerfinalJune2003.pdf (PDF)
