Current pose HUD indicator.
Consider displaying current player pose as a HUD indicator (i.e. silhouette)--it is sometimes difficult to ascertain current state, which is important in gameplay that revolves around stealth. Some corridors even force one to be crouched or prone.
This may be as simple as a profile body view of standing--crouched--prone. It could perhaps even be more complex and display a front body view that also shows leaning status (e.g. System Shock 1 HUD).