Bicycle Mechanics - Side pull brake maintainance

So what is the most important thing with side pull brakes.
I put new pads on and they don't stop well.
I put new cables on and they don't stop well.
I play a bit with the angles and they don't stop well.
I take it to the local bike store and it stops on a dime.

Can someone tell me the secret to a side-pull system.

There are a lot of little adjustments that an experienced bike mechanic can do to optimize side pull brakes. It sounds like you are getting close but just not nailing all the tricks to perfection yet. You said you are playing around with the angles: just what do you mean? Are you talking about centering the brake? , Toeing in the pads? Are you adjusting the pads to the rim sidewalls well? Are you using new housing? Did you retighten the cable,
I have to run out now, but, post back here more specifics of what you''ve tried and what kind of tools are you using to do the job?

The procedure is to put new pads on and seat them on the rim where I wish them to ride and tighten the Allen head. I went back out and played around with an old bike which the brakes never were very good and I got them to work good.
To me a person has to play around quite a bit to find just where they want to work good.
I played with my iron horse but I discovered there was too much play in the rear wheel so I took it apart.
Broken axle.
Have to wait till Tuesday till the bike store opens up.
Now the question is whether to just put in an axle or get the entire wheel.
After I fix the wheel then I can start playing around with the brakes again.
For a standard mountain bike, is there just a standard sized brake pad for the 26 inch wheel or are there different ones. It is almost like the pads I got are a bit to short allowing the cantilevers to lean in quite a bit.