A change in Turkish visa requirements from 10th April 2014 will mean it will no longer be possible to get a tourist visa on arrival when entering Turkey.
Applications must be made in advance, using the below link. A multi-entry visa valid for 90 days can be obtained for USD20 (subject to change).
Passports must be valid for 6 months beyond the expiry date of the visa.
Depending on passport nationality, additional requirements might have to be met.
Below link advises of these changes in more detail.