Wearing open shoes such as sandals means that you have a considerable amount of dirt and dust that accumulates on the footbed, and this means that you will need to have a way to clean them.
If your footbed is made of rubber, then you can toss the shoes in with the rest of your laundry and wash them with soap and water.
If you prefer hand washing them, you can use a brush with hard bristles and soapy water.
However, before you toss them in with anything else, make sure that you have removed the loose dirt on the shoes. This is especially true if you will be using a washing machine.
If you have a Birkenstocks footbed, make it a habit to wipe them down often to remove any accumulated dirt.
When they become quite dirty, you can use a paste of soda and water, rub it onto the sole and leave it soaking overnight. Rinse it off in the morning and air-dry the sandals.