Episcopalians cross themselves from time to time during the liturgy. This is not an unusual practice in Anglican spirituality. In fact, not crossing oneself is, globally speaking, much more unusual.
Crossing is an ancient Christian gesture, perhaps as old as the Fourth Century, in which one touches his or her forehead, heart, left shoulder, right shoulder and then back to the center of the chest. [Please note that it is Anglicans/Episcopalians who engage in that final gesture, as opposed to the form used in Roman Catholicism.]
There is also a particular way of doing so at the beginning of the reading of the Gospel. Perhaps this is something we can explore more during an adult forum or teaching liturgy.
The intent of crossing oneself is to mark the complete participation of the individual worshipper in the liturgy and in the life of the Spirit. In the gesture, we acknowledge that we engage head, heart, body, and soul. This is particularly important as Anglican/Episcopal spirituality has always advanced the total sensory experience of liturgy. We see the cross, hear the Word, touch the bread, taste the wine, and sometimes even smell the incense. This is also why the use of liturgical colors, flowers, and Christian art [such as is found in stained glass] is of such importance in our tradition.
If you are wondering when to cross yourself, there are times when it is particularly appropriate. Certainly, as we receive the blessing from a priest or bishop, it is customary to reflect the cleric's gesticulated cross of blessing by crossing oneself. It is also customary to do so at references to the resurrection in the Nicene Creed and Prayers of the People.
Although, I did hear what I think is the final word on this subject from The Rev. Dr. Thomas Talley, the late professor of liturgics at General Seminary. When one of my classmates, who is now a bishop somewhere in the Pacific, asked Dr. Talley when it was proper to cross oneself, Talley, who was from Texas and enjoyed good stories and sippin' whiskey, smiled and said, "Whenever the [Hades] you want to."
2/21 Update: Dear students and others in New Jersey, I see from the Feedjit live traffic stream that the burning question in the state of New Jersey today was "Why do we cross ourselves?" Really? A two-month-old posting has elicited that much curiosity? I wonder if it is a Confirmation class question somewhere or something from a parochial high school's theology program.
Kids, if you quote me, remember to footnote it.