My DD is only 2 years old (28 months, to be exact), and she really does a good job of behaving in church for the most part. I have always been a believer in keeping her in church with me from the beginning, so that she can learn how to behave in church (instead of going to the nursery and learning to act like a heathen! 
LOL!). She does pretty well. She has done much better since I resigned my position as worship leader and am therefore sitting with her thoughout the service. She has responded very well to my correction and training in church since that time, and she seems to be enjoying being in church more. This has really reinforced to me that I have made the right decision by resigning my position.
Anyway, at our new church things are a little more casual, and there is more room between the rows of seats for her to "roam." So sometimes she ends up roaming into the aisle next to the pews. I kept correcting her, and she would try to behave, but she just wouldn't be paying attention and would forget and wander into the aisle, or go dancing in the aisle.
Finally I came up with a plan! I found a roll of blue masking tape (the kind you use for painting, which removes very easily with no residue) and put it in my diaper bag. Now, if she is having a problem staying where she should, I pull out a piece of blue tape and place it on the floor and tell her not to go past that line. I usually will put one at each end, so she has very clear-cut boundaries. It works great. She knows not to go past the line and (almost) never does, and she will ask before crossing the line to go sit with grandparents or put in offering or whatever. Just having the visual boundary makes it a lot easier for her to know exactly where she is to stay. It works way better than a verbal boundary, and it keeps me from having to repeatedly correct her during the service.
It's great! Try it!