Elder flower (Sambucus nigra) is known to have diuretic, diaphoretic (induces sweating) and anti-catarrhal (dissolves mucus) properties
Infuse 2 heaped teaspoons of Elder flower in a cup of boiling water, steep for 5 minutes. Strain and drink several cups daily.
If not using the tea straight away, you can store it in a covered jug or container in the fridge for up to 24 hours and gently warm the mixture before drinking.