What if my mind wanders during dhikr?

At times people say that they are distracted and inattentive while performing dhikr, but that shall not be the reason to stop the remembrance. One may be heedless in invocation, which is far better than being completely and utterly heedless.

Allah, through His inaya, may grant invocation with attention to the one who is invoking His Name inattentively, and that is not difficult for Allah.

There is the account of the person who was a disbeliever and invoked the name of his godling, 'Ya sanam' (idol made out of stone), and while beseeching, by the slip of the tongue he uttered, 'Ya Samad' (Allah's attribute), and Allah replied, 'I am all Hearing, My servant'. The angels submitted to Allah that the person called upon His name only by mistake. Allah said that He knew it was by mistake, but the man invoked the name of his false god for years and the god could never reply, but when the servant invoked His name, He is the One All Hearing and He replied to His servant.

It is said that the person's heart was illuminated by the light and he accepted the truth. [Thanvi, The Road to Allah]

Remembrance increases the level of certitude, yaqeen, and one goes through the stations of ilm ul yaqeen to ain ul yaqeen to haq ul yaqeen. Initially it may be a struggle; then it becomes effortless and finally becomes a habit, then unconsciously one performs dhikr through the heart and the self is completely absorbed by dhikr.