To encourage as much English as possible in class, you should start as you mean to go on. You should give them the tools you think they are going to need.
Here are some ideas you can use.:
- Teach 'classroom language'
- Make them talk about themselves when you teach any new thing.
- Praise them.
- Allow exceptions for emergencies.
With older students have a discussion with them early in the course about why using English in class is important and negotiate with them about when L2 should be used and when L1 can be allowed.