In everyday life, we often have to show flexible behavior and switch between various tasks. Previous studies suggest that switching tasks is effortful and tends to be avoided. Therefore, the present study investigated whether and how explicit effort instructions would influence cognitive flexibility. In two experiments, we used a hybrid task-switching paradigm with a mixture of predetermined (forced-choice) and voluntary (free-choice) trials. Before every trial, participants received a cue that instructed participants to either invest high or no special effort. Voluntary switch rates (VSR) on free-choice trials and RTs and error rates on forced-choice trials served as dependent measures. Under the assumption of a bidirectional association between effort and cognitive flexibility during task switching, we expected increased flexibility following high-effort cues. In Experiment 1, using pictograms as cues, we observed no significant influence of the effort cue on flexibility (VSR or switch costs). In Experiments 2 and 3, using explicit words as effort cues, we found an increased VSR following high-effort cues. The results are discussed in terms of associative and motivational influences on cognitive flexibility.