Motivating others can be quite difficult if economic rewards or additional time off are not an option. Trying to convince you team that the overwork they are facing will be fun or exciting, is probably not the right way. But more than motivating people, the solution might be found in avoiding that they get demotivated, as this article explains.
Assuming that people are motivated when they start on a new job, there are several ways that help keeping up their motivation, without having to pay more or giving additional time off. As you can read in this article on zeitgeistmanager.com, most of these motivation methods are directly related to social skills.