Could you suggest a good warm up routine for the viola?

How I warm up for cello is I play a C major scale (2 octaves), then C# major scale (2 octaves), D major scale (2 octaves), and so on. It usually takes a while but it very much helps. Also if you don't feel quite warmed up after this, try playing C minor scale (2 octaves), then C# minor scale (2 octaves), etc.