Echinacea is very diurectic. Drink it as drops in a glass of water. But it can lead to a stomach upset if used excessively. Dandelion tea is also very good.
just want to point out that those drugs don't actually empty your bladder, they draw, water from your body through the kidneys and into the bladder. This fills the bladder and would normally trigger a need to go. Retaining urine in the bladder is caused by nerve and muscle problems and is treated in the first instance by inserting a catheter, then sorting out what is wrong.