Apply cleaner (e.g., pure citric acid) to the affected area and allow to sit for some time. Simply wet the affected areas with cleaning liquid (possibly soak a cloth in the liquid) and leave to soak in for a few hours. Afterwards, the dirt can often be removed very easily. As an alternative to citric acid you may use vinegar, vinegar essence or vinegar cleaner. Foaming cleaners have the advantage that they stick to smooth and vertical surfaces longer than liquid cleaners. Foam can also be sprayed directly under the rim of the toilet. Limescale, bacteria and germs can be reliably removed.