I've got some tips:
1.
Don't Wake Up!
You would not believe how many times I stumble upon this amazing cache of LEGO in my dreams. Just last night, I bought 3 huge tubs of LEGO containing Black Monarch's castle, Forestman's River Fortress, and tons of other sets from that era. Then I woke up.
2.
Mom
If you have a mom, aunt, or other local relative who goes to garage sales, ask her to keep an eye out for LEGO. My mother has called me on multiple occasions when she found LEGO at a garage sale, and I've gotten some amazing deals.
3.
Get the word out
Tell people that you are looking for collections of old LEGO. Perhaps your sister will marry a guy with a LEGO collection. Perhaps your friend's cousin is trying to get rid of his LEGO. Maybe somebody will trade you their old LEGO collection for other services; web programming, for example. Trust me, these things happen.
4.
Call!
Many Craigslist sellers do not respond to e-mails. Some are lazy, others don't check it enough, and some believe a buyer isn't serious if they won't spend the money to call. If the seller has a number listed, call it! They can't ignore once you start talking at their head.