You cannot carry forward deductions you forgot in a prior tax year; but you can carryover deductions that exceeded your maximum deduction limit. As in Publication 17/2014/Chp. 24:
You can deduct your contributions only in the year you actually make them in cash or other property (or in a later carryover year, as explained later under Carryovers ). This applies whether you use the cash or an accrual method of accounting.
Carryovers are covered later in that document:
Carryovers
You can carry over any contributions you cannot deduct in the current year because they exceed your adjusted-gross-income limits. You can deduct the excess in each of the next 5 years until it is used up, but not beyond that time. For more information, see Carryovers in Publication 526.
Publication 526 goes into a little more detail:
Carryovers
You can carry over any contributions you cannot deduct in the current year because they exceed your adjusted-gross-income limits. You may be able to deduct the excess in each of the next 5 years until it is used up, but not beyond that time. Your total charitable deduction for the year to which you carry your contributions cannot exceed 50% of your adjusted gross income for that year.
A carryover of a qualified conservation contribution can be carried forward for 15 years.
Contributions you carry over are subject to the same percentage limits in the year to which they are carried. For example, contributions subject to the 20% limit in the year in which they are made are 20% limit contributions in the year to which they are carried.
For each category of contributions, you deduct carryover contributions only after deducting all allowable contributions in that category for the current year. If you have carryovers from 2 or more prior years, use the carryover from the earlier year first.
But none of it allows for deductions left out of a prior year return.