Bhagavad Gita Chapter 8, Verse 13

ओमित्येकाक्षरं ब्रह्म व्याहरन्मामनुस्मरन् | य: प्रयाति त्यजन्देहं स याति परमां गतिम् ||

om ity ekākṣaraṁ brahma vyāharan mām anusmaran yaḥ prayāti tyajan dehaṁ sa yāti paramāṁ gatim

One who departs from the body, leaving it while chanting the one-syllable OM, which is Brahman, and remembering Me, attains the supreme goal.

Krishna gives the final teaching: 'Om ity ekaksharam brahma vyaharan mam anusmaran'—uttering (vyaharan) the one-syllable (ekaksharam) OM (om), which is (ity) Brahman (brahma), and remembering (anusmaran) Me (mam). 'Yah prayati tyajan deham sa yati paramam gatim'—who (yah) departs (prayati), leaving (tyajan) the body (deham), he (sah) attains (yati) the supreme (paramam) goal (gatim). This is the culmination: chanting OM, which is the sound vibration of Brahman, and remembering Krishna. OM is the one-syllable that represents the entire Brahman—all the unmanifest, all the eternal. Chanting OM at death helps you remember Brahman. Remembering Krishna helps you attain the Supreme Person. Together, they lead to the supreme goal. This is the practical method: chant OM and remember Krishna at death. This is how you prepare—practice chanting OM daily, practice remembering Krishna daily, so at death it's natural.