In Rolf Gates's and Katrina Kenison's book Meditations from the Mat, "Day 138" offers some good thoughts on perseverance:
Perfect firmness of body, steadiness of intelligence, and benevolence of spirit—that sounds great, but how do I get it? Stephen King says, "Writing equals ass in chair." I would go on on to say that asana equals feet on mat.
We encounter our first obstacle to asana practice off the mat—in the form of our endless array of reasons for not practicing today. I generally practice first thing in the morning for just that reason. As the day goes on, I am at increased risk of talking myself out of it. I know all the excuses and then some. And so I tell my students: Plan your week around your practice sessions; develop yoga buddies and make yoga dates; do a little yoga a lot, instead of doing a lot a little. Apply the yamas and the niyamas on the mat. Buy some cool yoga clothes, get the props you need, buy a good mat, make it fun. Go to weekend workshops if you can, take a yoga vacation. You don't need to make yoga practice your life, but do make it a part of your life. Give the opportunity you have been given the respect that it is due.
Especially relevant, and generalizable to many worthy pursuits: "do a little ... a lot, instead of doing a lot a little"!