There have probably been many discussions and articles on this topic but I just want to chime in with what I've observed after giving many English lessons.

Here are what I consider the four most common times people forget to add the:

1. the same - Most of the time, you need to add the before same. If you don't say the same, it would probably be a variation like this same.

        People say I look the same as her.
        These sentences have the same meaning.
        I keep hearing this same song all the time.

2. the UK/US - This applies to all variations such as the United Kingdom and the USA. It's not necessary in titles. There are other places, such as the Philippines and the Netherlands, but they aren't talked about as much in my lessons.

        Bruce Springsteen was born in the U.S.A.
        Will the UK still leave the EU?
        US President Barack Obama will leave office on January 20, 2017.

3. the internet (or the Internet) - When talking about the global network, you need the, unless it's modifying some nouns (you can add the if you need to specify) or you're talking about internet access (look, no the).

        Most people use the internet every day.
        Do you have internet here?
        He knows a lot about internet security.

4. one of the... (or a lot of the... or any other ...of the... pattern) - You have three choices. You can either use the article the before a noun, use a different determiner (e.g my or these) before a noun or use a pronoun by itself.

        He is one of the richest people alive.
        I waste a lot of my time.
        Some of us want to go home.

Feel free to add any other common cases and let me know if I missed anything or can make my explanations clearer.