I also have quite a bit of trouble with this. I used to use this heatmap script to motivate me to keep my streak going and not miss any days, but unfortunately that is now semi-broken because of a change by WaniKani, and can only track reviews on one device - which defeats the point a bit if you do reviews on more than one computer/phone.
So now the main motivator I use is my Summary Page script - seeing my accuracy and how many items I’ve reviewed does help a bit. And another very effective motivator is seeing the impending doom of 700 reviews piling up… 
But well, I do think it varies a lot for each person 
And things like @Akashelia says of having a specific task checklist could also be a great idea. In fact I might try that…