Customizing a practice room for your instrument is key (pun intended). In addition to the suggestions below, there’s a link to a great article from Strings Magazine at the bottom.
If you’re like me and can’t afford one of these ($5-15K) sound isolation booths, we do have some options. Read on.
Cover windows. Windows convey and reflect sound more than any other feature. Blackout curtains can work well. Or foam, cut to fit.
You’ll find more great suggestions over at Strings Magazine in this article by Greg Cahill: How to Soundproof a Room: Practice Without Bothering House Mates & Neighbors – Strings Magazine