Your skill practice points gradually decrease with time on their own. They will decrease slowly enough that if you are using a weapon infrequently but regularly, you won’t notice anything, but if you go many thousands of turns without using a skill, you may need to retrain and bring it back up to speed. There should either be a message when your skill falls a level due to this decrease, or when you try to start using the skill again only to discover that your skill has decreased.