A blessing is recited before donning the tallit or tzitzit (see Donning Tzitzit or a Tallit).
If one removes the tallit or tzitzit (for instance, if one needs to use the restroom during prayers1), with the intention of donning the same tallit or tzitzit afterwards, then it is not necessary to recite the blessing again when putting it back on. If, however, when taking it off there was no intention of putting it back on, or if a person wants to put on another tallit or tzitzit, the blessing must be repeated.
If the tallit inadvertently slips off, then the blessing must be repeated when putting it back on, unless at least part of the tallit remained on the body (catching it with your hand doesn't count!). If the person is in the midst of a section of the prayers where he may not interrupt, then he should put the tallit back on without reciting the blessing, and when he reaches a point where it is permissible to talk he should take the fringes of the tallit in his hands and recite the blessing.
If one puts on tzitzit in the morning before the morning hand-washing, then the tzitzit blessing should be recited afterwards, while grasping the fringes with his hands.
