1) One who is carrying a burden on his shoulder when the time for praying is reached if it is less than the weight of four Kav (5.5 Kilograms) or less, he can sling it over his back and pray. However, if it is heavier than this, he must place it on the ground for praying, as such a weight will disturb his concentration. (Shulchan Aruch and Mishne Berurah 97)
2) One whose Tallis started slipping off in the middle of the Amidah, may replace it. However, if it has fallen completely on the floor, he should not replace it as this will constitute an interruption. If leaving it on the floor will disturb his concentration, he may lift it up and put it back on, in between the blessings of the Amidah. (ibid)