You need to have some form of proof for any charitable contribution of any amount.
The rules for charitable donations have changed in recent years. For a cash donation of any amount, you must have a written record. That can be a canceled check, a bank or credit card statement entry, or a receipt from the charity. For a cash donation of $250 or more, you must have an acknowledgment from the charity stating the date and amount of the donation, and whether you received any goods or services in exchange for the donation. With no written record, you can not deduct a donation. For documentation requirements for non-cash donations, or out-of-pocket expenses when donating your services, see the documentation requirements in IRS Pub. 526.
Note that you can not deduct the value of your time or services as a charitable contribution.
The amount of your deduction for charitable contributions is limited to 50% of your adjusted gross income, and may be limited to 30% or 20% of your adjusted gross income, depending on the type of property you give and the type of organization you give it to. A different limit applies to certain qualified conservation contributions. These limits are described in detail in IRS Publication 526. See Pub 526 at this link:
http://www.irs.gov/pub/irs-pdf/p526.pdf
