While there are various tactics that we can try to keep our mind focused on the Rosary, one alternative way is to start off by asking God for help to keep focused.
Here is a brief prayer from the Rosary Guide for Priests and People that renounces distractions and implores God for aid to keep our thoughts on him alone.
I renounce all the distractions which may come to me during this holy Rosary, which I wish to say with modesty, attention, and devotion, as if it were the last of my life.
Incline unto my aid, O God!
O Lord, make haste to help me.
Glory be to the Father…
May I give honor to you, O Holy Virgin.
Give me strength against the enemies that oppress me.