I will use the term alignment to refer to the expanding the chest. We want to keep our ribcage lifted and expanded throughout the dance. A raised, open ribcage is necessary in order to maintain a good connection with our partner at the heart. A good practice to develop a sensation of good alignment is to interlace the fingers, or better still, grip one hand with the other, and then raise them above your head so that your upper arms are in line with your ears. The elbows do not have to be straight. Take a few breaths in this position and you will feel your chest naturally expanding.
Maintaining alignment during the dance
We want to maintain the sensation of an open chest throughout the dance. This may be difficult to do if we just have to focus on it but there is a workaround and it has to do with the tango estilo milonguero embrace: throughout the dance we keep the arms high at around shoulder level. In other words hold the elbows at around the level of the shoulders (see Floating).
Weak shoulder muscles
If you have underdeveloped shoulder muscles and struggle to keep your arms high for extended periods of time you can practice this by yourself regularly until your shoulder muscles develop and strengthen. Dancing usually requires some strength but is not in itself efficient as a strength building activity because that is not its purpose. Underdeveloped shoulder muscles can be helped by bodyweight training using the sun salutation or handstands, but in my own experience the most effective method to develop strength in the shoulders is the standing barbell press. This will lead to increased strength, given appropriate nutrition, at any age:
* I will reserve the term posture to refer to the head-neck position as it was specified by F. M. Alexander, that is, floating the head over the spine and lengthening the neck. Good posture is essential in everything that we do, and is necessary for easy and efficient movement without tension or constraint.