What are the ways to maintain presence of heart during dhikr, is it permissible to continuously say SubhanAllah and engage in dhikr an unlimited number of times, and is it permissible to send blessings upon the Prophet in the form: "Allahumma salli wa sallim ‘ala Muhammad sallallahu ‘alayhi wa ‘ala alihi wa sallam"?

What helps one achieve presence of heart during remembrance (dhikr) is contemplating the meanings of the remembrances, being mindful of Allah's awareness of us, and continuously seeking refuge in Him so that He may revive the heart. Continuous remembrance without a specific count is a good deed and an act of compliance with Allah's saying: "O you who have believed, remember Allah with much remembrance." As for sending prayers upon the Prophet, peace be upon him, with the aforementioned wording, there is no harm in it, though adhering to the Ibrahimiyyah prayer is more virtuous.