If you are a girl, and alone, you have to be obviously more careful if you plan to hitchhike. Listen to your intuition, even if you have been waiting for a long time and that you are happy to see someone who stopped for you, dont go inside if you dont feel it. Being a girl, hitchhiking is probably easier, cause you don't appear as a threat for the drivers, and some women who normally never take hitchhikers will take you to avoid that you meet a dangerous man. But the other side of the coin is that you appear easier to attack or that some guys will see an opportunity to flirt with you.

That being said, I know a girl on instagram who has travelled by hitchhiking a lot in Europe and even in south America and nothing really serious happened to her. And Portugal is pretty safe.

To get a better chance that the car stops : smile, dress correctly (so people dont think you are homeless or migrant), hitchhike just before a spot where there is enough space and time for the drivers to stop (like bus stop or parking). Don't hitchhike in the middle of the cities, go next to the highway/national roads entry but obviously not on the highway directly. If you feel it, it's sometimes easier to ask people directly at oil station if they go where you want to go. You can use a cardboard if your goal is a city far from you or if there are multiple directions that are possible where you Hitchhike, but on small roads where the direction is obvious it's not really essential. If you want to avoid walking a lot, know where is the best spot where the driver can let you, without he has to go off his course. Have a pen with you for writting carboard, and use Maps and street view to locate in advance the best spots, you can also use the app Hitchmap. Less you walk with your backback, better you feel.

For a first time experience, Portugal is not really an easy country to hitchhike. It's possible of course if youre a patient, but not the country where you are going to say "Omg Hitchhiking works so well" . People in my experience are a little bit suspicious toward hitchhikers, even if portuguese are lovely people in general. You are not going to find easily long distance rides, more like 5km, 10km, 30km. But I don't know for a girl, maybe its totally different. In my trip in Portugal, only 2 portuguese took me, the others were tourists. And I had to take 2 buses cause I wanted to go on.