I love trying new yarns and have to mostly shop online.
What I do is buy yarn I find so discounted, that it's cheaper than it's selling for on ebay. I figure that way, if it's not what I want, I can easily put it up for sale on the board or ebay and try again
This has allowed me to find out what types of yarns I really like.
Knowing what type of knitting you are going to be doing will help.
If you are knitting for yourself and don't mind handwashing.. that would open more options.
If you do a lot of gift knitting, you might want something else.
Knitting for children moves in a different direction.
For me, I like machine washable merino's for gifts.
Debbie Bliss Aran and DK weight merino's were on sale at Littleknits.com for about half what they were selling for on ebay. So I bought a fair amount
I read good things about Karabella Aurora 8. So I watched and when someone was destashing a few balls for about half retail, I bought them. I loved the yarn, so when I saw it go on sale for 40% off, I bought a bunch!
Watch elann.com preview tab. It tells you what yarns are coming up and when. Some sell out FAST. It's a chance to try some yarns without breaking the bank.
Look at blogs and see what people are using for what items that you like. Notice how the cables do or the drape looks... it can help you decide what to try.
The more you knit and try various fibers, the better idea you will have about what you like, and how those fibers tend to act under different circumstances. Think of it like learning to sculpt... and how different mediums will react
When I want to experiment with luxury fibers with abandon.. not thinking I have to find the perfect pattern because the yarn cost $$$$$ , then I use recycled yarns.
I love grabbing cashmere or angora or merino and just trying things without worry. I mean.. when I only have 99 cents - $5 in an entire sweater's worth of yarn, I can afford to gamble!
Just a few ideas.