I will keep this line in mind next time someone has some great unreasonable request for me, but please also remember that when someone has done you favours in the past, this line comes off as ungrateful.
I went over and fed my friend's cats when they were away; drove her to the auto insurance place when her car was totalled even though I didn't particularly feel like it and had to cancel other plans; lent her my camping gear when she didn't have any; found her a job when she lost hers, etc. etc.
Then it came time for me to go away and I needed someone to feed MY cat. She lived three blocks away. I got, "Sorry, that won't be possible."
I resented that. Maybe it really wasn't possible, but I had the feeling she just didn't feel like it. If she had offered some kind of explanation that sounded reasonable, I would have felt less like I'd been used all this time.