Just like any other income tax item, documentation should be there in case you are audited. Good sources of that are:

Credit Card Statements

EOB's (Explanation of Benefits) from each physician as this will have Dates of Service so you can do your mileage calendar.

Billing statements from insurance companies for premiums.

Pay stubs with a year to date figure for medical deductions.

Actual receipts

Drug stores such as Walgreens and CVS can provide an annual statement of prescription purchases.

If you do Online Bill Pay thru your bank, you can document payments to one payee during the year such as an insurance premium.

Hope this helps and thanks to DavidCPA for the bottom line on medical deductions.

Deb