In my case, I'm looking for very specific things for our next cattle dog. So, I can tell you how we chose a breeder for the next puppy we want to get (will be at least a year from now), but it is not general advice, but rather very specific. Only including it because it might help show how to focus on what is important to you.
For my husband and I, we fell hard and fast for cattle dogs after adopting our current girl, and getting to know the breed from the owner's side of the fence (they can sure be... not friendly looking from the stranger's side). The cattle dogs I grew up with were more like the show lines. That is, stocky and kind of wombaty in build. Our girl is leaner and more slender all over. It's a harder build to find, but can be seen in some working dog lines. Right now, this is the only place we are seriously considering buying from:
http://wanaberanch.com/heelersacd.html
'Handsome Henri" looks very much like our dog in build, but also in the personality showing through in photos of him. It has been specifically mentioned that he contributes to gentle and sweet mannered pups. We like that this ranch has working cattle dogs rather than show, since they are truly a working breed. Our two main considerations are always health and temperament rather than appearances, but we like how these dogs seem to have the first two, plus the body build we like best. We like that some of the pups are being used for therapy training, and that the dogs being bred are bred on a smaller scale. I love that you can visit the ranch, and that the people who own it seem genuine and friendly.
So to sum up, specifically to me I am looking for a very particular body build, and I'm looking for a dog from a truly working background.
On more general terms, I am looking at the health of the dogs being bred, the environment they are raised in, the ammount of care and attention being given to temperament and socialization, and puppies I can go to see in person. I do not want to go to the sort of breeder who will choose the puppy 'that best suits me'.
Of course...we may always end up adopting another adult heeler instead of buying a puppy. X) Anytime we see one in a shelter that is either purebred or has the heeler personality showing through in a mix, we just crumple up and have to fight not to take them home. The eyes look at you in that heeler way, and the body alternates between being shoved against the bars for petting and rolling over to explose the belly, or sometimes, they just hide in the corner (that was our girl...we had to work with her a ton to get her where she is today). But always, there is that eerie, eerie quietness...the only dog not barking in the entire shelter. Very nearly ended up taking one home who's time was up just a week ago, but thankfully there was a 'do not euthanize, dog being picked up tonight' tag on the cage.
Other people may be looking for additonal or different things such as:
-Conformation/breed standard
-Papers from particular groups
-Vaccinations/health care/genetic screening
-A breeder who is involved in showing (and winning)
-Possible training or free training lessons provided
-Possible agility or working trial titles
Etc.
Basically, I would take the answers you recieve and look at what is most important to you personally.