I don’t think that the people who say that literally mean that you can only write about things you’ve seen or experienced yourself. If that were true the list of existing books would be a lot smaller than it is today. There would be no fantasy or science fiction books for instance. Because let’s face it, none of us has ever seen an actual elf or alien before.
But it is true that writers use their experiences in their writing. If you write about something it’s useful to know what you’re writing, to do a little research. It’s also almost a necessity to use your own experiences to make your writing better.
With everything you write, take a moment to see if you can remember experiencing the same thing before. If you still know how you felt at that moment, or how something was done, it’s very useful to use that in your writing. For instance if you’re writing about a four year old child, it would help you a lot to remember what four year olds generally do and think and how they experience the world around them.
For those of you don’t have that good a memory, don’t worry. Just simply start to write things down now. If you don’t keep a daily journal (personally I’ve tried and failed several times to keep a daily journal.) just take a new notebook and keep it only to write experiences in.

Now later if you ever have to write about a character getting really scared and you don’t quite know how to do that: take out your experience book and read what you’ve written right after you were scared yourself. You’ll probably even remember the feeling while reading it (it helps if you also note why you were scared).
You can write about anything from playing in the snow, going through a breakup, swimming in a pond, crushing on someone, being ill or cold. I actually wrote a scene where my main characters were in a mountain range and one of them was coming down with a fever. As it happened, at the time I was ill at home with a high fever, feeling terribly cold but determent to write (lol). It really made me relate to my characters because I was basically writing down how I felt myself, only then in the middle of a snow storm in the mountains.
You could even write about something that we all experience almost daily: Being bored to death.

You all know the feeling of wanting to do something and nothing at the same time. If someone suggests something to do you don’t feel like it. We just hang on the couch watching terrible shows on TV. Write down why you think you’re bored. What you are feeling at the moment. How you eventually got over it. Plus writing that down will give you something to do so you’ll be less bored haha.
For those of you who go to another area or country sometimes in the holidays, don’t forget to write down what you feel,

Please comment and if you’re going to try this let me know how it works out for you!
Oh and if you'd want you can also follow me on Twitter! :D http://twitter.com/Lordkiwii
Keep writing!
Xx LordKiwii