The best solution is to use the same ratio for prints and image files instead of cropping.
For instance,
Bonus Print Pix (click on the flag nearest you for local service) offers traditional sizes (different cropping for each size), digital sizes (3:4 aspect ratio) and conventional sizes (2:3 aspect ratio).
Most low-end digital camera sensors are 3:4, while most upper-end cameras' are 2:3. I suspect the latter applies to you, so you could offer 12x18 instead of 11x14, 8x12 instead of 8x10 and 5x7.5 instead of 5x7 (BPP's prices are the same for conventional vs traditional vs digital).
Pierre