What matters when it comes to snow for March skiing:
Ski areas with higher elevations and more northerly latitudes can keep snow colder when thawing temperatures in March or during warming events degrade other resorts' snowpacks. Aspects have the same affect, so the snow at a ski resort with more east or south-facing slopes will suffer more quickly as the sun grows stronger into March and the later spring.
For that reason, ski resorts with more north-facing terrain will be able to shelter snow more thoroughly into and through March. The most important factors for good snow preservation in spring tend to be elevation and north aspects, so the ski resorts with the highest elevations and the highest percentage of north-facing slopes tend to be those that preserve their snow best. More explanation on this topic can be found on our snow rankings page.
Lots of east and south exposures coupled with cold temperatures at night can put ski slopes into a nasty freeze-thaw process, wherein the ski resort becomes an ice rink overnight and doesn't revert to being skiable until the sun thaws things out. If the sun doesn't come out, however, skiers can be stuck with terrible conditions for days on end.
So, quite simply, in March, look for resorts with high elevations and high percentages of north-facing terrain (~50%+).