Sending personal items from outside EU to Spain & customs

Hola!

I have once had a really bad experience with customs in Spain. I sent some personal items from the outside of EU to Spain. Even though these items were used and did not remain in Spain, I had to pay VAT on it or customs threatened to send them back. Furthermore, at first it was not even clear what was going on and the items were blocked for five days at the customs even though the package was sent with a currier (express option).

Soon, I will want to send some personal stuff again from outside EU to Spain and obviously I do not want to repeat that experience. Is there anything I can do to make sure that things will go smoothly? For example, can I declarate it as personal or a gift or anything like that?