I realise its annoying but be aware, not all ailments are visible - and that people who look ok may need that seat as much, if not more than, you do ...
it infuriates me when i get ignorant and snotty glares off people when i am sitting down on a crowded bus or train - just because i am young and 'look' healthy - i am not, my back and hips are agony most of the time, and i am as entitled to a seat as anyone.
so not only do i get the stares - i also know how you feel - when i am the one standing and in pain, and no-one moves for me, because i look ok, and have no stick etc ...