New material, perhaps something a bit easier, so that they can get on with it quickly. A change of style - pop, jazz, classical, something they haven't tried before.
Something very manageable and bite sized eg Dozen a day exercises - these always seems to go down well.
Change of approach - learning a piece by rote, making up a simple chordal accompaniment to a well known tune.
Duets are always good for providing a satisfying musical experience for the student.
Provide a performance opportunity - student concert, local festival etc.
Final throw of the dice for me is sometimes entering them for an exam, so that they have something definite to work towards.
Most important of all, though, is to discuss the situation with the student in a friendly and non-judgemental way. Find out what is going on in their life - are they bored, too busy to practice, disheartened, feeling that they have gone as far as they can or want to? Maybe it is time for a break, or even a change of teacher or instrument? We can't always fix the problem, but acknowledging that there is something amiss and showing willing to make some changes can be enough to show the student that something can be done and get them started again.