1) How the Buddha and the wise teachers motivated people thousands of years ago ?

The Bhagavad Gita defines Sattvic or a higher form of pleasure as "That which is like poison at first, but in the end like nectar, that pleasure is declared to be SATTVIC (pure) born of the purity of ones own mind due to Self realisation."

यत्तदग्रे विषमिव परिणामेऽमृतोपमम् ।

तत्सुखं सात्त्विकं प्रोक्तमात्मबुद्धिप्रसादजम् ॥ १८-३७॥

yattadagre viṣamiva pariṇāme’mṛtopamam

tatsukhaṃ sāttvikaṃ proktamātmabuddhiprasādajam

2) Hlada - Refined pleasure not born of the senses

Similarly, the hatha yoga masters like Swatmarama and Gheranda (authors of Hatha Yoga Pradipika and Gheranda Samhita respectively) knew that if hatha yoga were portrayed as a mere performance of austerities to attain the final state of liberation from the cycle of births and deaths (Samsara), it will not entice people to practice. Therefore, throughout the hatha yogic texts, phrases like "drinking the nectar" are used to emphasize the pleasure that comes from such yogic practices; and what is more, these pleasures that come from yogic practices are refined, are non-sensual (don't originate from the 5 senses) and are of higher quality. The term Hlada is sometimes used to denote these higher quality non-sensual pleasures.

In the chapter on mudras, The Hatha Yoga Pradipika says, one should revel in the bliss that comes from engaging the mudras.

3) The sequence of pleasure states in yoga

The Buddha talks about the Jhanas, the 4 states of advanced meditative absopbtion or Samadhi. These states are Joy, rapture, ease and equanimity. In this excellent dharma talk (audio and transcript available) given by Gil Fronsdal, he describes how Buddha when sitting under a rose apple as a child watching the fall harvest stumbled on the state of the Jhana (Dhyana in Sanskrit and Jhana in Pali), a state of pleasure and joy that is far superior to those felt from sensual experiences like eating, sex etc. The states always follow this sequence - joy coming first, followed by rapture or intense joy, then ease and finally equanimity.

There are Jhana meditation retreats conducted by people like Shaila Catherine (SF Bay area, California). The teachers usually say that one cannot usually enter a Jhana but you fall into it by chance when you are not seeking it.

In asana practice also, when the mudras are applied, by pressing on certain parts of the body, together with use of the bandhas and the Ujjayi breath, people sometimes get into exquisite states of mind that result in intense joy followed by a sense of equanimity and tranquility that they carry with them for the rest of the day.

4) The secret to maintaining a regular yoga practice

For some people, yoga practice, be it fasting (tapas or austerity), meditation or doing asanas, is a deep spiritual calling. These people do it because they see vast inner contentment from doing that practice. For those who do not have this calling, there are other means to sustain the practice year after year. This is done by exploring the subtle sensations of pleasure in the body. For example, as mentioned in the Gheranda Samhita and the Hatha Yoga pradipika, pressing certain parts of the body in asana practice using what are called mudras (seals), together with the application of the bandhas, results in exalted states of mind which are metaphorically described as drinking nectar.

Irrespective of ones level of practice, when one starts to identify these sensations, be they in asana or meditation, then one would be drawn to the practice because the practice, then, is no longer a chore but an act of joy and rest. While falling into these states may not happen initially, repetitive focus on the subtle sensations in the body, over a long time will more likely result in falling so. In fact, knowingly or unknowingly, this is why many people get up in the wee hours of the morning to either meditate or do asana practice. This they do, forsaking some of the lower pleasures like socializing into the late hours of night, as shown in this video by Richard Freeman, jokingly titled "Yoga destroys your life" (....your samsaric life, the lifestyle that results in a seemingly never ending births and deaths).

References

1) Hlada: Pleasure not of the senses (How the great teachers of ancient times motivated people in their yoga practice)

2) Chathur Dhyana- The 4 Jhanas : An overview of the Jhanas described by Buddhism

3) Awakening through the Jhanas- Importance of the deep meditation states

4) An excellent Dharma talk and transcript on the Jhanas by Gil Fronsdal