As far I practise this way:
Chinese health balls - I use them when travelling or on my way to somewhere. I got this tip from my friend who plays saxophone and I must say that it really helps my fingers get stronger and improves coordination. And I do not mention other positive sideeffects

Lips practising with pencil - my flute teacher gave me this tip - have a pencil and use only your lips, no teeth, to move pencil up and down.
I use these whenever I can, despite the funny look I may sometimes produce

Mind practises - sometimes I think about scales, tunes... etc and project their fingerings on to the flute.
I would like to know, if there are some other tips out there
