According to Lonely Planet, these are twenty important things to consider when traveling solo. I added a few comments.
- Safety first-I think this applies, solo or otherwise.
- Learn the language-the basics are always useful and helps make a connection.
- Talk to locals-of course, this is one of the main reasons to travel!
- Look for shared housing options-I am not sure about this, though many friends do this often.
- Go your own way-the perfect travel partner gives and gets their own space.
- Challenge yourself-always include some of this, but not 100% of the time.
- Experience it all-pace yourself, what is the big hurry?
- Believe in yourself-enough said.
- Try couchsurfing-not a big fan, but it does work for many travelers.
- Take your time-especially as I get older!
- Be your own photographer-of course.
- Get lost-intentional or not, it leads to good stories.
- Join a tour-be selective!
- Ask for advice-ask the people who have been there.
- Work for room and board-go ahead!
- Join a cruise-go ahead!
- Expand your skills-a cooking class, or ??
- Make wise choices-of course!
- Trust your gut-of course, but be prepared.
- Embrace total freedom-always, this is the essence of travel.
I have added a few tips:

- Always carry some local currency, do not rely totally on your plastic.
- Always have a Plan B.
- Make a friend or buddy.
- Don’t stay out late!
- Always carry a paper map, do not rely on google maps.
Here is a case for solo travel:
There’s Never Been a More Necessary Time to Go on a Trip Alone (msn.com)
And for the best places in the US for solo travelers:
The survey’s (Vacation Renter) entire top 10 U.S. cities for solo travel were:

- New York City (not sure I agree)
- Chicago (definitely great for solo travelers)
- Los Angeles
- Washington, D.C. (ditto, as this city has lots of singles)
- Las Vegas (surprising, though maybe I hang with a different group))
- San Francisco (maybe)
- Denver (I would agree)
- Austin
- Seattle (probably, though best to avoid the hordes of homeless)
- Phoenix (particularly in Scottsdale, home of plastic surgery)
Best places in the world for solo travelers, based on my experience:
Athens
Amsterdam
Bangkok
Munich
Mexico City
Vancouver
Buenos Aires
Sydney
Queenstown
Costa del Sol
I have found that solo travel opens your possibilities in many ways. One, I am more open to new adventures. Two, I always make new friends. And three and most important, I might discover something new about myself!