(Some at my previous day job once remarked that it would be cheaper to order me to do tutoring than to pay for a freelance English tutor, given the hourly equivalent of what Japanese salarymen my age made versus the typical price of an hour of English instruction in this neck of the woods.)
Edit to add:
That aside isn't quite so useful for folks not here. Let me hum a few bars: students at highly competitive suburban high schools, students studying for the LSAT, working professionals with gaps that are impeding their career growth, and middle class women who are filling a hole in their life through learning stuff all pay rather substantially more than 10 year olds who need help with multiplication.
The thing about a) is that often the kids aren't stupid but they've just gotten a bad explanation. I find that a one on one with a no-nonsense explanation and enough examples clears everything up and suddenly you're a hero. The thin about b) is that you might need to know things specific to the type of thing you're helping with (eg, SATs) but you're more likely to get repeat jobs from the same customer.
The marketing scheme here is mostly word of mouth. You can put up signs and that'll get you some business but once you successfully help your first couple of students the word spreads like wildfire between stressed parents and you suddenly have a reputation. Your reputation is very important so don't scam your clients, work on giving a good impression and maybe even give occasional freebies to regulars.
I also know a number of guys that make acceptable money teaching chess at private schools, homeschool groups, etc.
Another friend does something similar teach biology to groups. He also does guided walks and field trips for groups at local parks. He has a collection of interesting animals and even does birthday parties on the weekends. I also know a guy that's an animal trainer that now makes all his income from school events and birthday parties.
Or you should read about The Great Zucchini, a birthday party clown making $300 a party. I'm sure the business has gone downhill since 2006, but it's still good money for a flexible, weekend-only job.
Anyway, it can be very rewarding and fun work. Once you get that first student, it becomes exponentially easier to find the next by asking for referrals.
Anyone know if something like this exists?