I have an Indian tampura machine which I use to practise my intonation. If a piece is written in C major I set the machine to generate a C pitch. When there are a lot of modulations I don't use it. This way you don't have to use the open strings as a pitch reference. So you know that you are playing in tune (or not!) and that you don't shift the whole pitch somewhat. A lot of times you shift the pitch a little without really noticing. At least I do sometimes. I don't practise like this all the time. Especially when playing in the thumb position it can be helpful I think. What do you think of this practise method?