1. Expand your searches -
You'd be surprised what slips under the radar when you give too specific search directions. Sometimes, shops don't name their products "pink belt" or "blue button down shirt." Sometimes, that style of skirt is called "Jezebel Skirt" or "Paris Night Ring." So, when you're searching, don't limit yourself to very specific things. You never know what you're going to miss.
Don't agonize over it, but some colors just don't match. Different tones of red and different materials are going to be fairly obvious since they all tend to be crammed into a small space, not spread out over your frame. You're most likely to avoid this by looking for items made by the same manufacturer since they're making from the same cloth for some clothes and are generally using the same lighting. Of course, some colors (I find this to be particularly difficult with velvets) just aren't going to match up in the item that you want, but for most items you're going to have a lot of pages to dig through so there is usually a new item hidden on the next page that can fit. If this fails, remember that you can always clip items yourself using the Polyvore clipper, and some of those will be the best items you can find.
3. Join and submit to groups
There are a lot of groups out there dedicated to Goth style. A few that I am particularly fond of are Go Goth, New Age Goth N Punk, Old School Goth, Gothic Group, Into the Shadows, Gothic, Victorian, and Lolita, though I suggest you stay away from groups like EMO_GOTH_H0T_TOPIC. Or maybe a group that pitches with the tagline "IF YOUR EMO OR GOTH THAN THIS IS THE PLACE FOR U...OR IF U JUST HATE PREPPY PEOPLE...OR IF U JUST LOVE HOT TOPIC TO DEATH..." if your thing. I don't know. Point is, if you join one of these groups, others in the group will be able to locate your sets more easily and you will get more feedback on them. That, and a group will be sure to provide you with a lot more inspiration than you might otherwise have.
4. Add non-clothing items
While Polyvore is generally used for clothing sets, you can add more than just the standard clothing items. Add make up, backgrounds, furniture or other set pieces, wigs, text, and just about anything else to really finalize your vision for the outfit. One of my favorite things that I've seen (and used myself) is outfits that use the swatches of makeup at the background instead of just setting the tubes or tins off to the side, so get creative with those.
5. Consider a model
There are a lot of models to chose from, but unfortunately many of them come with clothes on previously. You're going to be lucky to find a model with an intact head and arms, but more likely you're going to find a usable head and no arms. This is where it gets more complicated. You may have to take arms from another model, generally from a woman modeling either a tank top or some sort of gloves, and crop those behind your top. Try to match the skin tone as close as you can. If you're having trouble finding a head that you like, search for wigs and hair accessories. Most of them can be found on someone's head. Apart from that, I've seen Polyvore sets use dolls and even animals to model the fashions, so don't feel limited to a human face!
Bonus tip: Don't fall into the lazy trap and keep the backgrounds for pictures! Your sets will look their best if you take a few moments to clip the background out.