One school of thought is that microtubule-associated proteins bind to microtubules and induce a conformational change that makes the microtubule more stable.  This is accomplished by reducing the frequency of catastrophes, increasing the frequency of rescues, and decreasing the rate of microtubule depolymerization.  Together, these factors make the overall population of microtubules much more stable.

The regulation of the tau molecule is acheived through expression, localization, and phosphorylation.  Tau is primarily expressed in neurons, although other non-neuronal cells can have trace amounts of tau.  Within neurons, tau can be found in all compartments of the cell, but in different phosphorylation states in different compartments.  This is important since tau's ability to stabilize microtubules can also be controlled by site-specific phosphorylation.  It is thought that phosphorylation of specific residues, especially in the microtubule binding repeats can modify tau's binding to microtubules and therefore prevent it from stabilizing the microtubule.  Another level of control can be acheived by the developmental regulation of the expression of tau isoforms and kinases.  Throughout development, the ratios of different tau isoforms to kinases and phosphatases can change, leading to different states of tau regulation.

In addition, it is thought that other structural modifications can affect the tau molecule, especially in disease states.