Inspiration waxes and wanes for practitioners of all types. That's normal. However, many of the teachers I know talk about this and teach that motivating yourself to meditate even when you don't really feel like it is also an important part of the path. I would guess the tradition you practice with has some standard contemplations one can mull over to inspire oneself.
In Tibetan Buddhism, we have what is called "the four mind changings," which refer to contemplating the preciousness of a human life, impermanence and death, Karma, and the suffering of samsara. Contemplating these subjects are foundational for Tibetan practitioners, and as far as I can tell there is no discrepancy between this practice and any other stripe of Buddhism, as it is based on the Four Noble Truths. This is what I do to kick myself in the ass when I feel sluggish. I don't always convince myself but it does help. If you're interested, there is a bottomless wealth of teachings on the four mind changings accessible online.
Also, I second Ayu's experience that finding a teacher does wonders for meditation practice.
I am a lustful, angry dullard with no power of realization. Do not put anything I say into practice without first confirming it with a qualified teacher.