This is similar to 'licence' and 'license'; it depends upon whether the word is used as a noun or a verb. In both cases, the 'c' form is correct for a noun, while the 's' form is used for a verb.
So, these are all correct (because a noun is used):
Target practice
Sharp practice
General practice
Practice makes perfect
(Similarly: TV licence, wedding licence, etc.)
However, these are also correct (because a verb is in use):
Practise what you preach
Practise the piano
(Similarly: "to license an event").
All of the above refers to British English.
US English uses the 'c' form of 'practice' for both noun and verb. (Perversely, Americans use the 's' form of 'license' for both noun and verb).
Chris