A general advice is to hitchhike with an umbrella in case of rain. It keeps you dry for a long time. On petrol stations it remains easy to approach drivers to request rides. Statistics say that in the Netherlands - a country generally known for "frequent rain and wetness" - rain is falling 4% of the time. In general people highly exageratte the badness of weather - perhaps just a way to explain themselves why they wish to drive as much as possible by car.
On motorways during rain it is mostly a matter of going from one petrol station to the next and waiting and requesting rides under the roof of the petrol station. Only a strong wind could still make one wet.
When you stand next to the road and it starts raining, your chances that somebody stops quadruple for a couple of minutes until you are wet, and then your chances slide down. Therefore, keep smiling when it starts raining and the first few drops come down.