A friend was asking about swaddle blankets and I mentioned it to Kit and this was the email he sent in response to her questions. I married the Swaddle Pro. 😀 Seriously, he does all the swaddling around here because I always do something off and the baby escapes. He's also the master at burping, diaper changes without waking the baby up, and getting the baby's to take nice, long naps on his shoulder.
I've had way too much time to think about swaddling over the years. I hope you don't mind if I add some things to what Heidi was saying.
In no particular order:
Swaddlers need to be a fairly stiff fabric, very non-elastic. I've had the best results with natural fibers -- cotton blankets are especially nice. Too much synthetic in the blend will stretch and slip and they tend to be hotter.
I didn't like the storebought swaddlers with the little velcro tabs and all. They focus all of the stress of the swaddle at one or two points and invariably come all undone except at those two points. Fairly useless, IMO.
Bringing me to the real secret of swaddling: surface area. If you get as much contact with the blanket over as much of the length of the baby as possible, then it'll stay more snug. Bonus points if you take that first pass around the baby and secure the end behind the baby's back so they're laying on the point of the blanket -- lots of surface area.
It also helps if the child's arms are wrapped up without a point of leverage. If their arms are crossed in front, they'll totally wiggle out. If their arms are more at the side, they have a harder time of wiggling.
Here are more tips from Kit, including his swaddle demo video!