This is something I get asked about all the time. In fact, it's hard to get away from ads promoting something designed to give you a 'quick fix' for your posture. 'Should I get one of these posture straps to help my posture?'. Given the difficult nature of postural change, could such things be a magic bullet to stop neck and back pain? Unfortunately while they may work as a correctional tool for some, these devices often disappoint.
These devices promise a gentle corrective push in the right direction, working without you even having to give it a second thought. However, this ignores how the body processes and learns movement. All those bad habits that have built up over time influence at an elemental level how we initiate and complete movements through our limbs and spine. Simply strapping things in place and applying movement will not guarantee a change in movement, in fact if mindful change to those movement habits is not made such devices actually provide strength training to the already overly active and tight muscles producing the undesired posture. So, the net result is that those over tight muscles get stronger not weaker.
So, how should we correct the posture? Unfortunately the only way to balance out the muscle pull around the shoulders and back/neck is through active movement training to strengthen up, both neurologically and physically, the weaker muscles. Now, this is where a strap such as the one pictured above can come in useful by helping you to actively engage the muscles you are training through your rehabilitation exercises. Used as such, it can be a gentle reminder when you are drifting away from the desired position. Note that this is much different from merely strapping yourself in and forgetting about it. This is still part of a process of mindful change and the strap is merely an early warning system used to keep you on the straight and narrow.
So, should you invest in one? Maybe, but only if you decide if it's going to be a part of your overall treatment plan. If you're unsure if it could be, then ask your osteopath or physio how it could help.