Time will do it for some people, but not for everyone. For instance I come from the north-east of England and had a very strong accent which people outside of the area struggle to understand. I then lived in other parts of England for several years and the accent slowly went away. I now have a fairly standard English accent that everyone can understand.

I know other people from the area that have lived away from home for 20 years and still sound exactly how they used to!

Also my accent comes back when I speak to people from "home".

My Spanish also improved with alcohol and a few months travelling around south america, where people are far less picky about the language than they would be in Spain and not too many local people speak English.