And of course the real trick to 'practising' is to not practice the bits you can, but the bits you can't! That is, don't just play the whole piece thru endless times - first play only the bits that are the hassle, then integrate them slowly back into the easier bits.

E.g., on the whistle the octave jumps are murder at first: working on them for a while will actually improve your overall breath control technique on the instrument as a whole.