Your appearance is important when hitchhiking!
The first and most important details are the clothes you wear. Try to wear bright colored, well visible clothes. This is extremely important during the night or in bad weather, but also during sunshine. A Yellow or Orange Shirt/Pullover/Jacket works best. If you don't want to spent much money on new gear, a reflective warning vest pulled over your jacket will perfectly do the job. This is also extremely important while walking on a dangerous road trying to find a good spot.
Obviously there is a difference between hitching in Spain in the summer and in Scandinavia during wintertime. Nevertheless, it is often smart to buy some good outdoor stuff. It is light and dries very fast. There is nothing worse than a wet jeans or cotton-shirt if it's bellow 10°C. Plus, you can wash it and it dries fast. Outdoor wear differs a lot in the degree it adopts smell. You can wear some for days without smelling, others stink after two hours hiking.
Hitch hiking is an outdoor sport and it pays off to wear good shoes. They might not be best for dancing when you reach your destination, but it is important while walking along the road. You will need to cross rocky areas, puddles, mud, snow etc. Sometimes you have to be quick and step off the road due to big trucks etc.
Avoid dark or military clothes, they do not create a lot of trustfulness. Looking too much like a colorful hippie will possibly scare other drivers off. Take off your hat (unless it's a really funny one), don't wear gloves, open your jacket, even though you might freeze.
Show both your hands and, if possible, forearms. Don't wear a hat or sunglasses.
Your mood is very relevant. So always be happy, or at least pretend to and keep smiling. This will get you a ride much faster, than if you're looking tired and unmotivated.
As a guy, try to shave before you hit the road. It'll reduce your waiting time. When hitchhiking in foreign countries it might pay off to look like a foreigner.
You are more likely to be picked up when you look somewhat decent and clean. Wear fast drying, easy to wash clothes, avoid cotton.
Avoid stinking! This might not be the easiest part while hitch-hiking on long distances, but remember: You are not the last hitch hiker this driver might pass! Having a stinky passenger in his car will make the driver think twice before picking up someone again. This will make it easier for the whole hitch-hiking community! Carry a second gear with you, wash your clothes regularly, use fast drying gear. Some outdoor gear adopts smell faster than others, ask in the store or inform yourself before buying.
Use deodorants. Clean your shoes of mud before hitch hiking (not in front of the eyes of the driver).
Look the approaching driver in the eyes! Smile!
As a Girl, don't behave and look too salacious. This might give a wrong impression.
If you've been waiting a long time it might be good to do something funny, like waving at random cars, or play some small musical instrument. Jokes will not be heard, and may make you look insane.