Picking up hitchhikers
When considering picking up hitchhikers, keep in mind first and foremost, you are not obligated to let someone into your car just because you pulled over. That being said, try to be welcoming, but at the same time alert and cautious. Just because someone looks bedraggled, unshaven and questionably clean, doesn't mean they are dangerous or shady. You have to consider that this person has very likely been on the road for days on end, probably without a proper place to sleep, and maybe without a proper meal. Of course, it's also not uncommon to find properly showered and shaved hitchhikers wearing a suit and a tie.
A good starting point for your judgment is often their luggage. You should take a good look at the person's belongings as you slow down and pull over. Do they have a well worn rucksack and are their hands full with a map and a book while they frantically try to grab everything and rush toward your car? An honest traveler will probably have similar paraphernalia. Still, many hitchhikers hitchhike for short day trips on well-known territory and sometimes don't carry any luggage at all.
After this initial look, you should exchange a few words with the hitchhiker before throwing the door open. Many drivers pull over and hurriedly gesture for you to pile in, but you can often tell a lot about a person from an quick exchange, which could save you a lot of trouble. During this quick exchange, you have to keep in mind that the hitchhiker may also have apprehensions, and generally, they get the privilege of asking "where are you going?" before the driver does. This is because a the hitchhiker needs a "way out" if they pick up a bad vibe from the driver and decide they (politely) want to decline the ride.
If you decide that you'd rather not have this person in your car, don't feel like you have to just because you've pulled over and had a few words with them. Your instinct is your best defense mechanism and ignoring it because you feel awkward is a bad idea.
If you've decided that the hitchhiker is indeed legit (and most are), it's probably best to get moving so you don't fall victim to the even more likely danger of being hit by another car. Some more cautious (maybe paranoid) hitchhikers may be unwilling to part with their belongings, in case they need to make a quick escape. You could ask if they mind putting their pack in the trunk if you have limited room, but if at all possible allow them to keep their stuff nearby.
If, before you let them in, you ask them where they are going and they say a place that is very very far away, but you are only going to a place that really is just 20km down the road, don't just drive off as it is quite likely that they would very much appreciate the extra 20km that you can take them. Even one km can be a big help if you happen to know that it is a better spot (e.g. a big gas station along the highway) than the one they are in.
A better spot is also more important than how far. Taking someone from a good big gas station and putting them on the highway (or off the high way) might make more problem than helping. As long as you can put the person at a somewhat better or the same quality place a little bit of a ride is better than no ride, if not much else it gives the hitchhiker a new scenario and some hope.
In any case, it's good to show some sign of recognition, even if you don't plan to stop. A quick smile or a gesture indicating that you're only going locally can give a hitchhiker some hope. And many hitchhikers love human interaction, so it's nice to have a chat at a gas station.
Sometimes you encounter hitchhikers at traffic lights. If the traffic light is followed by a convenient place to stop it's best to go there. Otherwise you have to trust on your instinct.
Highway on-ramp, bus stops
Bus stops are convenient places to stop. Highway on-ramps can be more tricky, but are similar.
It's great when it happens, but hitchhikers don't need to go to their final destination with just one ride. Usually a hitchhike trip takes many consecutive rides. To avoid missing out on cars that go shorter distances people often put two or even more destinations on their sign.