1) Order overstocked or discontinued yarn online
About a month ago I found an online yarn store called DBNY. They have deals where you can buy what they call "bags of yarn." You can get 5 balls of yarn for $5 or 10 balls of yarn for $10.
You can find this website here: http://www.discontinuedbrandnameyarn.com/shop/
2) Always check the sale section of online yarn stores
Though the sale section doesn't always have what I want at the moment, I sometimes stock up on yarn I might need in the future. This can save lots of money. Knit Outta the Box currently has yarn on sale for 50 cents! (needless to say I bought ten! haha)
You can find this $.50 yarn here: www.knitouttathebox.com
3) Sign up online for emails and updates from multiple yarn stores
By signing up for emails, you'll be reminded whenever the yarn store has a sale. Sometimes you can even get codes for certain percentages off! This can save loads of money while yarn shopping. I've signed up for emails from KnitPicks.com. They often have great sales during the holidays.
You can see this store and sign up for emails here: www.knitpicks.com
4) Use coupons
I don't know if you guys are addicted to Hobby Lobby like I am, but I go there for yarn often. I never go to Hobby Lobby without taking their 40% coupon that you can print anytime. Whenever I'm near to Hobby Lobby, I stop by and just buy one ball of expensive yarn with the coupon. This can save lots of money!
You can print the HobbyLobby coupon here: http://www.hobbylobby.com/weekly/coupon.cfm
5) Always check the clearance section at yarn/craft stores
Hobby Lobby also has a section where they keep clearance yarn. Sometimes they don't have much but other times they have really nice yarn half-price. Whenever I go, I get some yarn on clearance and save it to use when I need it. (just don't buy yarn you don't think you'll use.)
6) Look for yarn on Etsy
When I first decided to look for yarn on Etsy, I was a little skeptical. Though I knew that Etsy would have some very high quality yarn, I thought the prices might be ridiculous. If you type in "Yarn Sale" on Etsy, you can sometimes find great deals though! Change the page so that the lowest prices show up first and then scroll through till you find some yarn you like. I also love being able to help out small businesses like mine.
Here's an Etsy store with awesome yarn and great sales: http://www.etsy.com/shop/brokemarys
7) Become friends with other yarn obsessed people
There are many perks of being friends with people who love yarn as much as you do. You can trade yarn, sell leftover yarn to each other, borrow knitting needles, etc. I don't have a particular friend I do this with, but many of my friends know I love yarn so they buy it for me as gifts. People occasionally give me their unused yarn as well (It feels like Christmas!).
Hope these tips were helpful!