How to improve my "ths?"?

When I was working in an ESL cl, I noticed a lot of students had trouble with this. Place the tip of your tongue just on the edge of your top front teeth when making the sound. It will keep your tongue in your mouth better. If you have a friend who you don't mind asking to do this, have them say the sound while you watch their tongue position. Then practice in front of a mirror. This made a big difference for my students.