MAKE NEW FRIENDS
Strangers are just friends you haven't met yet.
Let's be real. People are flaky, and no amount of Head and Shoulders in the world can cure your friends from reneging on their promises. If you want to travel, and your friends can't make it with you - consider going solo.
STAY IN A HOSTEL
It's common to be friendly amongst travelers, because people are generally happier when their not hunched inside their cubicle crying into their keyboard. Hostels are the perfect socializing place to make new friends. Though it may be nerve racking at first, you'd be surprised how far a smile and a "Hello" can get you. Other travelers are also a wealth of knowledge, and some of the best advice I've gotten has come from asking other travelers where to visit. If you're nervous about staying in a dormitory room, many hostels have private rooms so you can get the social benefits of a hostel with your own privacy.
JOIN A TOUR GROUP
Free walking tours, bike tours or any sort of tour will force friendship with others due to the close vicinity. When you think about it, it's almost like being at the first day of school again and meeting new friends.
You can use TINDER to meet people. I used TINDER abroad to ask locals for suggestions on places to go. Obviously, if you are going to meet with a stranger in a foreign country you should be careful, so if you do meet with someone use your judgement and meet in a public place. Girls who are traveling solo can join a charter of GirlCrew to meet other girls to go out with.
UTILIZE YOUR NETWORK
Asking your friends to link you up with people you already know is a good one, maybe even a Facebook post asking friends if anyone has suggestions or friends. I've had friends who met up with locals on couchsurfing.com.
TALKING TO STRANGERS IN PUBLIC
Stay off your phone, it makes you look busy. If you're in a hostel and there are a group of people chatting, my go to ice breaker is, "Sorry to interrupt. Does anyone know the wifi password?" From there, you can turn the conversation any way you'd like. "Thanks! I'm just trying to look up some fun places to eat around here. Do you know of any good spots?" This way, you're engaging and not being rude or creepy. You could just as easily say, "Hi. I'm _. Where are you from?"
People tend to be very social in hostels. In a tour group, I just say, "Hi" or comment on something about the tour like, "What did he just say?". In bars, just standing by the bar is an open invitation to talk to the person next to you. "Is the bartender ignoring you too?" These are just examples of things you could use if you are a shy person, but ultimately - traveling is all about getting to be you with no inhibitions. So, enjoy traveling solo and getting to be exactly the person you want to be.
HAVE AN OPEN MIND
You'll meet people from all walks of life, and you shouldn't judge someone because they look like a nerd or any other reason. At the same time, don't worry about trying to make friends the whole time. Enjoy being on your trip and the friends and conversation will come naturally. In Rome I was so nervous about being alone that I didn't get to enjoy it as much as I should have. Traveling solo has been one of my favorite experiences and it's taught me that I am capable of doing a lot more than I ever thought possible. Enjoy it.