Practice or Practise?

A LITTLE TRICK TO SPOT 'PRACTICE'

Try using the word 'preparation' (or 'lessons') instead of 'practice'. If the sentence still makes sense, then 'practice' is almost certainly correct.
(This trick works because 'practice' is a noun, just like the words 'preparation' and 'lessons'.)

A LITTLE TRICK TO SPOT 'PRACTISE'

Try using the verb 'to prepare' (in its various forms; e.g., preparing, prepared, prepares) instead of 'practise'. If the sentence still makes sense, then 'practise' is almost certainly correct. However, if you find yourself using 'preparation' then you should be using 'practice', because both are nouns.
(This trick works because 'to practise' is a verb, just like 'to prepare'. )

NO CONFUSION

There should be no confusion with 'practising' or 'practised'. These are always verbs.

PRACTICE IN AMERICA

Whilst many in the US have adopted 'practise' as the verb and 'practice' as the noun, it is acceptable to use 'practice' for both noun and verb in American English.

I must keep practising/practicing that accent.
(both versions acceptable in American English)