Don't keep the biggest just cause they're the biggest.
Don't keep the fattest
Keep ones that were born sooner in the season, as their mothers bred early
keep ones out of good mothers - breed back time, easy keeping, temperament, size, feet, bag, no horns, etc
keep ones that look like a good little heifer, not a steer
keep ones that are friendly
keep ones that are the colour you want
don't keep any that you don't really like, just because you want more heifers and they're 'good enough'