When trying to predict crowds, think about school calendars. Clearly, Christmas and New Year are the same for everyone and will concentrate the crowds. This past season, many schools did not reopen until January 7th because of how various holidays lined up. Not that I remember major crowds at Squaw then.
Martin Luther King weekend is rather universal. So is Presidents' Day. Less so is Presidents' Week, which some schools have dropped or substantially scaled back. Spring break varies widely and often isn't an issue as other activities compete. Often it's just the Saturday of a 3-day that is a zoo.
Take a look at the black-out dates for the cheaper season passes. That will tell you when crowds are expected. I've heard, however, that some resorts get more crowded than others during those times. Specifically, I've seen more crowds one week before or after a blacked-out weekend at Squaw. I've heard, though, that Northstar is packed during "prime" weekends.
As JayT suggests, any mention of snowfall in the news brings people out of the woodwork. Fresh powder, especially on a weekend, brings them out in spades.
The best antidote for crowds is to ski where most others cannot. Take lessons and you'll improve. Take lessons on a crowded day and you'll bypass the lift lines. Learn the layout of the resort and watch how the crowds move; go where they just came from.
My family and I did 30 days at Squaw this year, including early January, MLK, Presidents' Week, etc. I really have no complaints, but it was a low snow year. If you have the option, sleep close to the hill. Often the morning drive and parking are the worst part.