1. Don't make saving money a prerequisite.
Nobody is renting out their place in order to save you money. You get what you pay for.
2. Book early.
The good hosts in prime cities book up months out. You don't want to be in a situation where you have limited choices and have to lower your standards.
3. Only book with hosts who have many positive reviews.
See above, and never break this rule.
4. Try to stay at least a week.
It's business. When you represent a grand or more to your hosts, they treat you accordingly. But when the hassle inherent to renting to you is barely worth the $200 or so you're paying, service will suffer.
Any time I've broken any of these tenets I've wished I'd just booked a hotel.