Can you build up a tolerance to herbs and begin to need more to get the same results?

There are very few herbs that present this problem.  Echinacea is one that I have heard from various sources loses effectiveness in a few weeks.  Stimulant laxatives such as senna, cascara sagrada, rhubarb root and aloes will weaken the intestinal muscles leading to a laxative dependence.  And yeast can build a tolerance to pau d' arco.

If so, what is the best way to prevent this?

I don't really bother with echinacea in the first place.  It is way over hyped and too unstable.  So I stick to stronger and more stable herbs.  I also advise not even using the stimulant laxatives.  As for the pau d' arco it is best to avoid long term use or to cycle the pau d' arco with other herbs that have the same properties.  Mathake is one of the herbs often cycled with pau d' arco for yeast infections, but it can be hard to find.  Although, pau d' arco is rarely ever required long term anyway.  If someone has chronic yeast infections the focus should be on building the flora to control the yeast, not trying to kill it directly.

Also, a manicurist recently commented on how healthy and strong my nails are. Is this more about hydration (maybe from my fish oil supplementation and wearing gloves when I do dishes), or about vitamins (I do take b-complex twice a day)? Or, perhaps, something else?

The primary thing that will strengthen nails is silica, which can be derived from various foods, herbs or diatomaceous earth.  Maintaining stomach acidity also helps by increasing silica absorption.

The B vitamin biotin also strengthens the nails.