you can marry in mild steel plate alongside each rafter and bolt it through - that makes it very strong and rigid. Alternatively you can use ply I am told. I suspect though, you might be meaning you want to strengthen the celing joists, but dont want to assume - more info please - I have done this conversion twice on my own home and enjoyed the results very much! both times I had to lower ceilingsa though as it was a 1920 Californian Bungalow so the roof pitch didnt allow us to stand up completely.
....catchy phrase here