As a practitioner and a Yoga teacher, I believe that there are three factors that are important to stay consistent at Yoga asana practice:
1. Be comfortable with repetitive movementsOur minds seek novelty and people who are new to Asana practice tend to get bored after a few days. When someone goes for jogging or gymming, though the physical movement seems repetitive, novelty is brought in either by listening to new podcasts, audiobooks, music or other forms of entertainment to keep the restless mind occupied. Yoga is all about awareness and staying mindful during the practice. Similar movements done consistently and slowly yield amazing results in terms of strength and flexibility.
2. Stay there for the long run
Because of initial enthusiasm, we tend to push ourselves more and strive to do challenging asanas earlier without giving enough time for the body to open up. This increases chances of injury and strain. It is better to take it slow and continue our practice in the long run. I'd rather prefer to do 12-rounds Suryanamaskar comfortably in my 70s rather than push myself to hit the 108-rounds in my 40s and end up straining my lower back.
3. Adopt an attitude of "I'm grateful that I can...."
If your body is relatively stable without any major ailments, approaching Yoga asanas with an attitude that "I'm grateful that I can do Trikonasana or Sarvangasana or Padahastasana...." helps you to stay consistent with your practice. We take our flexibility and mobility for granted, but when diagnosed with certain ailments, the same poses which we could have comfortably performed earlier, now become contraindicative. For someone with high blood pressure or lower back related issues, forward fold poses are to be avoided.