Deferment and Forbearance from Federal Student Aid (studentaid.ed.gov) provides the information you need to understand the options you have when you are having trouble paying your loans.
It is important to take action before you are charged late fees. If you have trouble making your education loan payments, immediately contact the organization that services your loan. You might qualify for a deferment, forbearance, or be able to change your repayment plan. For Federal Perkins Loans, contact your loan servicer or the school that made you the loan. For Direct and FFEL Stafford Loans, contact your loan servicer. If you do not know who your servicer is, you can look it up in the U.S. Department of Education’s National Student Loan Data System, NSLDS.
Note to PLUS Loan borrowers: Generally, the eligibility requirements and procedures for requesting a deferment or forbearance for Stafford Loan borrowers also apply to you. However, since all PLUS Loans are unsubsidized, you'll be charged interest during periods of deferment or forbearance. If you don't pay the interest as it accrues, it will be capitalized (added to the principal balance of the loan), thereby increasing the amount you'll have to repay.
Military Service related options for Limited Interest Rates, No Accrual of Interest, and Deferment of Student Loans from Federal Student Aid (studentaid.ed.gov)
Military deferment forms: