most desperate and last used method is to pause mass production in mass extractors, but you should not forget that ths method exist...
some people tell that they force mass stalls to prevent power stall (start t3 mex and heavy assist it) but i would not recommend that way
also if you have unfinished structires you can reclaim then and get mass and
power back
also you can
1) selfdestruct building
2) start rerbuilding it on same place so it will be started from 50%
3) instead of continuing building start reclaiming it and you will get 50% power and 50% mass back
you can reclaim enemy unfinished structures to get power too
and ofc best way is to predict power stall and make it never happen by taking in account how many engineers you are useing for building and assisting stuff so if you want to begin spamming factories with 6 engies you will 1rst need about 9 pgens just to begin and then add 2 more for each factory own consumption
that helps creating better build orders
better build order usually means - get more factories earlyer
so if you make second land with just acu you build 5 or even 4 pgens before starting second fac, but if you assist acu with engy to boost your build order you will need to add at least 1 more pgen before second factory...
also if you are on the edge of power stall you can put away assist from air factories, but still keep factories producing, cause they will benefit from adjustency bonuses (ofc if you have power gens around factories)
also some times if you have firebase you can build t1 pgens around t2 shields and radars there if you suddenly about to go power stall
also need to keep in mind that some units are made with enabled cloak/jammer/shield abilities
and if for some reason you have many of them they may suddenly affect your power
i am taking about cybran t3 air scout and t3 gunship and monkeylord
and UEF t1 frigate
units with personal shield are kinda obvius
Edit: OHH how could i forget!!! you can reclaim forest if you need power, as well as small bushes, but they are kinda not worth reclaiming...