Found those useful so I copy them here for future reference:

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Introduction to string arranging - a nice tutorial, more on the theory and composing/ arranging side.

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CineBrass Impressions - Good tips for brass writing.

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Tutorials on AudioTuts+
- Making Your MIDI Strings Even More Realistic
- Quick Tip: Realistic Panning
- Making a Convincing Melody with Sampled Orchestral Strings