Measures in place during repair to prevent breach of data

Maybe encription of all sensitive data stored on your hard disk will work.

You can use the Mac OS version of Truecrypt 7.1a (tha last one known to be uncompromised)

While not an official Apple answer, the document below offers some useful and pertinent comment.

http://apple.stackexchange.com/questions/72417/can-i-trust-apple-stores-employee s-with-my-laptop

Back up all data on the internal drive(s) before you hand over your computer to anyone. There are ways to back up a computer that isn't fully functional—ask if you need guidance.

If privacy is a concern, erase the data partition(s) with the option to write zeros* (do this only if you have at least two complete, independent backups, and you know how to restore to an empty drive from any of them.) Don’t erase the recovery partition, if present.

Keeping your confidential data secure during hardware repair

Apple also recommends that you deauthorize a device in the iTunes Store before having it serviced.

*An SSD doesn't need to be zeroed.