In traditional fung shui, mirrors don't open up spaces in rooms or replace missing elements. They reflect back what is captured in them - whether it's good or bad. Mirrors can't choose the type of energy they reflect. That's why hanging one on the wall directly opposite your front door will result in missed opportunities - as favourable and unfavourable energy is deflected. A small ba gua mirror placed above your front door, on the other hand, will deflect attacking energy from passageways and sharp corners.
In living and dining rooms, mirrors can be used to increase the yang energy as long as family members don't sit with their backs to them for long periods. The view the mirror reflects should be positive as it's better to have beautiful mountains and harbour views reflected into your home rather than old buildings or your neighbour's home.
Some masters recommend placing small mirrors behind stoves that stand directly opposite an entrance so the cook can see who enters. Full-length mirrors that reflect the whole stove, though, will cause instability in the food.
Dressing-table mirrors should never reflect the bed as this may affect marital relations. If possible, place a mirror on the same wall as the headboard. Alternatively, hang one on the inside of a wardrobe.