But magic is never as simple as people think. It has to obey certain universal laws. And one is that, no matter how hard a thing is to do, once it has been done it’ll become a whole lot easier and will therefore be done a lot. A huge mountain might be scaled by strong men only after many centuries of failed attempts, but a few decades later grandmothers will be strolling up it for tea ‘and then wandering back afterwards to see where they left their glasses.
–Maskerade by Terry Pratchett

I hope Rincewind never catches up with me

The root problem, Rincewind had come to believe, was that he suffered from pre-emptive karma. If it even looked as though something nice was going to happen to him in the near future, something bad would happen right now. And it went on happening to him right through the part where the good stuff should be happening, so that he never actually experienced it. It was as if he always got the indigestion before the meal and felt so dreadful that he never actually managed to eat anything.
Somewhere in the world, he reasoned, there was someone who was on the other end of the see-saw, a kind of mirror Rincewind whose life was a succession of wonderful events. He hoped to meet him one day, preferably while holding some sort of weapon.

–Interesting Times by Terry Pratchett

“It’d never work,” she said. “We’re basically incompatible. When I’m 5′ 4″ you’ll still only be 3′ 9″. Anyway, I’m old enough to be your mother.”

From Witches Abroad:
It’s a strange thing about determined seekers-after-wisdom that, no matter where they happen to be, they’ll always seek that wisdom which is a long way off. Wisdom is one of the few things that looks bigger the further away it is.*
* Hence, for example, the Way of Mrs Cosmopolite, very popular among young people who live in the hidden valleys above the snowline in the high Ramtops. Disdaining the utterances of their own saffron-clad, prayer-wheel-spinning elders, they occasionally travel all the way to No. 3 Quirm Street in flat and foggy Ankh-Morpork, to seek wisdom at the feet of Mrs Marietta Cosmopolite, a seamstress. No-one knows the reason for this apart from the aforesaid attractiveness of distant wisdom, since they can’t understand a word she says or, more usually, screams at them. Many a bald young monk returns to his high fastness to meditate on the strange mantra vouchsafed to him, such as ‘Push off you!’ and ‘If I see one more of you little orange devils peering in at me he’ll feel the edge of my hand, all right?’ and ‘Why are you buggers all coming round here staring at my feet?’ They have even developed a special branch of martial arts based on their experiences, where they shout incomprehensibly at one another and then hit their opponent with a broom.