April 15 may not be the most pressing financial deadline this spring.
For families with college-bound students, university deadlines are approaching fast for filing the Free Application for Federal Student Aid (aka FAFSA) and other state and private college aid forms.
Each year, billions of dollars get left on the table because families don't apply for aid, or fill out forms incorrectly, Carolyn Berkowitz, president of the Capital One Foundation, told attendees at a mid-January financial aid panel in New York City.
Getting forms in correctly and on time can make a big difference in your out-of-pocket costs for college and how much your child may receive in loans. Average aid awarded during the 2012-13 school year was $13,640 per full-time student, including $7,270 in grants that do not need to be repaid, according to The College Board. During that same academic year, the average cost to attend a private four-year college was $40,261, including tuition, fees, room and board, and $18,171 for a public four-year college.
(Read more: Coming up with $500,000 for college)
One of parents' big mistakes is waiting to file financial aid forms until they have prepared their taxes. Many colleges set January or February deadlines for aid applications. Miss them, and there may be less or no aid by the time you submit, said panelist Adam Stevens, college advisor for scholarships and financial aid at Brooklyn Technical High School.
Tardiness could even affect acceptance. "Students don't realize that [meeting the deadline] is another way to show you're serious," he said.
—By CNBC's Kelli B. Grant. Follow her on Twitter @Kelligrant and on Google.