Removing finger marks and general grubbiness from your lovely painted walls is generally easy and effective.
Firstly wipe marks with a damp cloth as and when they appear.
For large areas of grubby wall firstly vacuum to remove dust then use a solution of warm water and a few drops of washing up liquid, sponge over the whole area and dry with a clean soft cloth.
Heavy staining can be tackled with soda crystals or sugar soap.
If the grubbiness has got too bad, spot paint or even repaint whole of wall.
If there is mould on the walls (particularly susceptible in bathrooms and kitchens) wipe bleach over area and leave for an hour or so, then wipe clean with warm water. Dry with a clean cloth.