Heathur, I had this same problem at first before I figured out that it's all in the makeup. Face shape and body structure are more basic things that you can fiddle with for free as you need to, but skins can be very expensive.
Things I learned:
* The cheap skins (< 1K$L) are often not worth it if you really want to look good. There are exceptions to this.
* If the clothing looks ill-fitting on the poster, chances are it's ill-fitting on your avie.
* If the poster appears shoddily made, be wary of getting quality product.
* Most of the store display posters won't really give you a good idea of the skin tone and makeup look you'll end up with. One, avatars are different and two, the lighting used to create the poster (as well as the light in the store) can make every difference in the world. (I'm keeping a really dark tanned skin I bought that I absolutely hated when I tried it on -- it's more than just tanned, it's black!)
* Look for stores that let you demo the skin for free. It'll say "demo" all across your body until you remove it, but at least you get a better idea of coloring and makeup.
* Don't settle for the first skin you come across. Take notes. Rename your landmarks in your LM folder to note to yourself where you saw something (after 50 stores, they start to look alike). Hold out for the better skins that are between 1K and 1500 $L and offer you choices of makeup (they're out there). I could be wrong, but I haven't seen a $4K skin yet that I thought was much better than the $1k skin I wear. (When I get richer I'll check out my theory

).
* Ditto on the hair. There was a thread here a week ago about the best hair.
Here's the thread -- I've personally had great luck with Lil's hair and Munchflower, along with a few others.
Mostly, I'd say just go with trial and error. Some more experienced players might be able to steer you to the better stores selling skins and hair, but in the end you'll probably just stumble on one someday that really defines "You", and you will have found it all on you own .
Cindy