The best way to assist at the Mass is whichever way helps a person to pray best. Missals are good because they help you to know what is occurring and being said during the Mass, but it profits you nothing to just mindlessly read along, becoming annoyed when you fall out of pace with the priest, not contemplating the mysteries. So yes, you can use a missal to pray during the Mass, but the key there is to pray, not just follow along. Countless prayerbooks have been published with good private devotions during the Mass, prayers to direct your intentions. There is also, of course, the pious custom of praying the Rosary during the Mass. These are all well and pious practices, but the key is to actually pray, whether that be following your missal, praying the Rosary, or some other pious set of devotions. So pray the Mass in whichever way benefits you most, but take care to actually pray.