Willpower can be strengthened by strong decision and continuous training to avoid desires and do good which goes against our desires. Fasting, Performing Wudhu during cold weather, Performing Night Prayer, Performing Fajr Prayer on time are few examples of the practical ways to strengthen willpower. In general, willpower needs training to be stronger whenever we have options of either to follow our desires or our intellect.
There are many Du'as in this regard e.g. Du'a Kumail (QAWWI ALA KHIDMATIKA JAWARIHI WASHDOD ALAL AZEEMATI JAWANIHI
قَوِّ على خدمتك جوارحي واشدُد على العزيمة جوانحي