Essentially the hollandaise will break when either the clarified butter you've emulsified with the egg yolks separates, or due to excessive heat the yolks scramble and turn into solid, lumpy parts.
If you're afraid your yolks with curdle, lift the pan up off the heat and keep whisking. Or add your butter more slowly. You can also reduce the heat on your double-boiler (if you are using one).
If the yolks and butter are just not retaining their emulsified state, whisk in a little boiling water (like a teaspoon) during the emulsification process. Or just stop adding butter, keep whisking, and start again. If you store the hollandaise at too high of a temperature, it will break down.
I find the more you heat/whisk the yolks before you start adding butter (so the yolks become light and creamy in color/texture) they will take to the butter more efficiently. If you want to cheat, you can use a robot coupe. Add your yolks and seasonings into the mixer and let it go. The friction heat will partially cook the yolks. Then slowly add a stream of clarified butter into the feed tube. The heat from the butter will cook the sauce the rest of the way. (Assuming it is still near-boiling hot.)
Finishing the sauce (much like a buerre blanc) with a very small amount of heavy cream will give it more durability, but don't go nuts with this.  | 
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