The authors find that ego depletion is lessened if you believe that willpower is not a limited resource. Abstract:

Building cognitive abilities often requires sustained engagement with effortful tasks. We demonstrate that beliefs about willpower–whether willpower is viewed as a limited or non-limited resource–impact sustained learning on a strenuous mental task. As predicted, beliefs about willpower did not affect accuracy or improvement during the initial phases of learning; however, participants who were led to view willpower as non-limited showed greater sustained learning over the full duration of the task. These findings highlight the interactive nature of motivational and cognitive processes: motivational factors can substantially affect people’s ability to recruit their cognitive resources to sustain learning over time.

NY Times article: http://www.nytimes.com/2011/11/27/opinion/sunday/willpower-its-in-your-head.html?_r=0

Recent study by the authors: http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0038680

A related, slightly older study by the same authors which reviews several experiments: http://intl-pss.sagepub.com/content/21/11/1686.full

(Not sure if the second link is openable for everyone; I have access to most things through my university library.)

The NY Times article seems to be saying that studies showing an increase in willpower after consuming sugar are incorrect in some way. What I'm left wondering is: what if you believe that willpower is unlimited, AND you ingest glucose shortly before doing a difficult task? Would that be even better?