Sometimes I have little motivation, so I do several things to get myself motivated.
1 - Listen to dhamma talks, read about the benefits of meditation
2 - Use a guided meditation for my sitting, the structure and guidance helps when one is unmotivated
3 - Read some short suttas before sitting, especially ones which have to deal with the shortness of life or the importance of practice (the mountain simile for example)
4 - Reflection on death and mortality, it is not hard to find videos of death and dead bodies on the internet...I'll leave that to you guys to find them on your own....
This is pretty powerful stuff, so if you have a weak stomach just think about your own mortality abstractly.
5 - Find a teacher, sangha, buddy - this is very powerful as well, if you're doing it alone, you don't have to.
Vayadhammā saṅkhārā appamādena sampādethā — All things decay and disappoint, it is through vigilance that you succeed — Mahāparinibbāna Sutta
Tārakā timiraṃ dīpo māyāvaśyāya budbudaḥ supinaṃ vidyud abhraṃ ca evaṃ draṣṭavya saṃskṛtam — A shooting star, a clouding of the sight, a lamp, An illusion, a drop of dew, a bubble, A dream, a lightning’s flash, a thunder cloud — This is the way one should see the conditioned — Vajracchedikā Prajñāpāramitā
I have seen all things that are done under the sun, and behold, all is vanity and a chase after wind — Ecclesiastes 1.14