I grew up in South Florida and every home we had down there had at least 1 sliding glass door. If we were ever locked out of the house, the first door we would try to get into was one of the sliding doors because they could be easily shaken and pushed off their tracks. My father used to take broom stick handles and put them down on the bottom tracks to keep the door from pushing open as easily, put if you tried hard enough, you could still wiggle the door open most of the time.
My mother in law moved into a retirement community a few years ago and her apartment there has a sliding door. She is on the second floor but still felt uncomfortable with it being unsecured. We thought about putting something in the track on the bottom to block it like my dad used to do, but she would never be able to bend down and lift it up when she wanted to open the door. My husband and I went to Lowes to see what else there was out there to use.
What we purchased was the Charlie Bar Sliding Door Security Bar. It is an aluminum bar that attaches to either side of the door frame in the middle part of the door. The bar can be adjusted to fit doors from 36 to 48 inches. Installation was easy and required only drilling a few holes on either side of the door frame for the brackets and then screwing them in place. Because the bar is located in the middle of the door instead of the bottom like the stick method, the door can not be rocked out of place. The bar is also very simple to use. It goes across the door and snaps into place on the other side. To open the door, just push up on the bar from the inside and either lock it into place on the other side or let it hang straight down.
The bar works great for her and gives her the added sense of security she needed. If I ever have a house with a sliding glass door again, I will defiantly be getting one of these for our home as well.