Sufi & Bhakti approach: “Acceptance (of the Will of God) is the key to experiencing a positive state of mind. We must practice acceptance throughout the day.” This theme is explored in Sufi & Bhakti Poetry.
Advaita Vedanta approach: “Non-attachment is the key to experiencing a positive state of mind. We must practice non-attachment throughout the day.” This theme is explored in Who Am I?.
Buddhist approach: “Equanimity is the key to experiencing a positive state of mind. We must practice equanimity throughout the day.” This theme is explored in Awareness and Equanimity.
Acceptance, non-attachment and equanimity are the same concept.
The main practice uses the word 'all' with respect to physical and mental phenomena. What does the word 'all' mean in the above practices? Each tradition refers to 'all' physical and mental phenomena. Understanding 'all' is challenging. Some meditative practice is essential to be able to see / witness / experience 'all' physical and mental phenomena in third person. There are hundreds of meditative practices. Two common techniques are 'japa' (mantra-based / chanting-based meditation) and 'breath + insight meditation'. These two techniques appear to be different. But do they converge as progress is made in either practice? I think so … but I don't have sufficient personal experience (I'm not sufficiently skilled in either technique) to explain any further.
Learn meditation: No matter what system is being followed (Sufi / Bhakti / Advaita / Buddhism), meditation is necessary. I'm familiar with only the meditation technique which is taught in the 10-day Meditation Course. This course first teaches breath meditation, then insight meditation. There are 'mantra'-based techniques ('japa') which are advocated by many Sufi & Bhakti saints. I'm not familiar with these techniques and I don't know of any reference text which describes them in detail.
Validation: How may we verify that the main practice actually works! It is important to verify. Over time, as we continue to apply whatever technique we've learnt, we must keep probing inside and figure out: is it really true that I experience a positive state of mind when I accept (when I'm non-attached; when I'm equanimous)? Is it really true that I experience a negative state of mind when I don't accept? In order to discover these answers, we have to build an 'internal barometer' to gauge our current 'state of mind' — is it positive or negative? Building this 'internal barometer' is a challenging step in itself. Such a barometer can be developed with the help of meditation.
Boundary: What exactly is the boundary between 'I' and 'The Universe'? Understanding that boundary is super-helpful in identifying what all has to be accepted in order to experience a positive state of mind. For those who prefer a rational, structured approach, Advaita Vedanta provides a beautiful conceptualization of this boundary. See Who Am I?. For those who connect with an emotional, devotional, fluid and poetic approach, see Sufi & Bhakti Poetry. The boundaries are the same in both.
Refinement: Acceptance, non-attachment and equanimity are tricky concepts. What do they really mean? Talking to a teacher, talking to friends and reading books is helpful.
Measurement: How may we figure out whether we are in a state of acceptance or non-acceptance at any time? What are the signs? How does acceptance or non-acceptance manifest itself in our personal behavior? Can we make measurements of some aspect of our body or our mind to figure this out? Or do we need somebody else to make this measurement and make us aware?
Tools, Techniques & Practices: Once we realize that we are in a state of non-acceptance, what can be done about it? While we are in a state of acceptance, what can we do to prevent non-acceptance from occurring? Are these tools and techniques to be applied once, or repeatedly throughout our life?
Community of like minded friends: It helps to keep in touch with a circle of friends or a community which help us strengthen our practice. In Buddhism, this is called Sangha. In Sikhism, this is called Sat-Sangat or Sadh-Sangat (congregation of saints or seekers of Truth).
Keep practicing!