Your appearance is important when hitchhiking!
Putting together an excellent outfit doesn't just change the means by which that other people perceive you, it changes the means by which we experience ourselves. Think for a minute about just how you're feeling when you place on your favorite shirt, as well as exactly that pair of jeans for just right. It changes your entire personality.
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 or jacket works best. If you don't want to spend 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.
To limit the amount of clothes you take with you; use layers. In the morning you start with several layers, as it gets warmer you take them off and at night you can wear them again. It's great if you can adapt to your driver's air conditioning, or the sudden rain or 4 km tunnel when you're in the back of a pick up truck.
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 wet jeans or cotton-shirt if it's below 10 °C. Plus, you can wash it and it dries fast. Outdoor wear differs a lot in the degree it adopts smelliness. You can wear some for days without smelling, others stink after two hours hiking.
Hitchhiking 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. At times you might 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. Also helpful are t-shirts that identify you with something, be it a country, a football team or a band - Zenit has been picked up because of his "Switzerland" t-shirt more than once.
Sometimes, your clothes give away that you're a traveller/tourist, because the people on the street where you are usually wearing something else than what you wear. As mentioned above, it might scare people off if it is too extreme, but on the other hand it might also get people interested to look like you've stories to tell.
Show both your hands and, if possible, forearms. Don't wear a hat or sunglasses. However, is if it is just too hot and sunny to stand it with no protection choose a hat over sunglasses, as the latter hide your eyes and this makes most people suspicious. Wear a short-brimmed straw hat or a light coloured cap as long as it doesn't hide your face too much. If it is raining, it's better to use an umbrella than a hood.
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.
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 thing while hitchhiking long distance, but remember: You are not the last hitchhiker this driver might pass! Having a stinky passenger in the car will make the driver think twice before picking up someone again. This will make it easier for the whole hitchhiking community! Carry a second set of gear with you, wash your clothes regularly, use fast-drying gear. Some outdoor gear picks up odours faster than others, ask in the store or inform yourself before buying.
Use deodorants. Clean any mud off your shoes before hitchhiking (not in front of the eyes of the driver).
Most service stations on the highway have showers. Use them in the morning if you start your journey after you camped at a service station.
Hygiene can save your health and your life; it can prevent many sicknesses.
Look the approaching driver in the eyes! Smile!
If you're likely to be perceived as a woman, don't behave and look too salacious. This might give the 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. Hitchhiking a bike or a motorbike will also cheer you up easily—they always look so funny showing that they would love to take you but don't have the space.