What are the symptoms exactly?
If the discs are warped this can be caused by your braking style, as has been mentioned, but it can also occur if the hub mating surface isn't spotlessly clean and free from any dirt or rust when you mount new discs. Cheap non-branded pads can cause problems too.
But there are other causes of vibration during braking. If the callipers are sticking a bit and not completely free then you'll get some vibration. Also if there's any brake debris preventing the pads from moving freely. Or if a calliper or carrier is loose at all.
Another thing to check is that the calliper slide pins are completely free to move, and well greased with an appropriate lubricant. If not this will cause uneven pad pressure and hence vibration.
Wear in the suspension and steering can do it too, as can tyre problems.
Lastly, are you fitting decent branded new discs? On stuff like this the motto "buy cheap, buy twice" was never truer!
Jack
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