Whenever I watch a long series, such as One Piece or Detective Conan, I do research to find out the order of the tv episodes, movies, specials, etc. Usually, the movies come out after a certain amount of regular episodes. After I get to the movie, I watch it, then take a break from that series for a few days. I find that by having a "break" from watching the anime will allow me to watch the rest of the series. When determining when to take my break, I think of how long my attention span is, and if a series is 800+ episodes long and ongoing, such as Detective Conan, I will watch up to 20-25 episodes since that is usually how long my attention span is for most series.