Here is some things I do when I go to the cattle sales.
Sometimes sale barns will have special sales along with their regular sales. Find out what may be selling before the day you go. There is nothing like showing up at the sale barn to buy feeders on a day they are having a cow and calf sale. (been there, done that)
As it was mentioned before, take notice of cattle you want may be interested in. Write down their pen numbers or individual numbers. Also, make a note of cattle you do not want to buy; the ones that look sick or injured.
Select a weight range. Feeder cattle should be around the same weight range.
Learn the graded scale. Here in WV we have L, M, S frame size, then 1, 2, 3, 4, 5 mass size. For example, L1 & M1 being better cattle than M3 or S5. I think the grading system varies for each state.
Take a calculator. Check to see what the cattle in a particular weight and grade range are selling for that day. Price seems to be change all the time.
Check prices on the cattle you plan to buy before going to a sale. You can find this information on your state agriculture web page. Here in WV they have information on the state sale sites and other large national sale sites. The site also gives futures on all types of cattle.
http://www.ams.usda.gov/lsmnpubs/CFAWV.htm
http://www.ams.usda.gov/lsmnpubs/NatSum.htm
Hope this helps.
bcox