Maybe vegas' solution helps when a Mac has to be sold, it does surely not help, when the mac has a problem (e.g. you cannot start the system anymore) and has to see the service point.

So, I prefer the following:

No personal data on the HDD where the OS sits. Just links to a 2nd HDD (e.g. for Mail, address book, and so on)

If this is not appropriate, e.g. for 'working data' on the Desktop: encrypt it with openssl

So anytime, the Mac can go anywhere with a very, very little risk that data gets in the wrong hands.

Lazy

I know this is a bit off-topic. Another "solution" is to use a spare drive with a basic system configuration which you can swap for your working drive anytime the computer needs to go to the shop. Simply take out the drive with the important data and plug in the spare and send to the shop. Your data stays with you all the time.