1. Materials - Halloween decorations generally come in one of five materials: paper, foam, rubber, plastic, and metal. Right away, I will tell you that it is generally not a good idea to buy Halloween decorations made out of paper and foam because they are not meant to last very long. They will tear and chip and will generally not be a good use of your money. I would also avoid from buying anything made out of rubber because they tend to be Halloween-specific decorations that look cheap or cheesy. (I know, we all love rats, but I have yet to see someone successfully incorporate a bag of rubber rats into Goth decor.) This leaves plastic and metal decorations, which are by far your safest bet for long-term wear.
2. Motifs - This one is more up to personal taste but it is generally accepted that while there are many elegant and Gothy ways to decorate with Halloween items, hanging a banner that says "Happy Halloween!" on it isn't as elegant as it is...silly? Feel free to disregard this rule if you're making a Halloweentown-themed room or similar. For the rest of us, sticking to motifs such as spiders, coffins, vampires, etc. will look less "Halloweentown" and more "Gothic abode."
3. Quality - The quality of the items has a lot to do with the previously mentioned materials that they are made out of. Most people have their Halloween decorations up for a maximum of about three or four weeks so the items they buy for the holiday don't have to be particularly good quality. However, if you're intending on buying them for year-round wear, you'll quickly find that they deteriorate faster than you intended. Thus, you should be picky about what you're buying. Beware of items heavily coated in glitter (it will shed on you like nobody's business) and candles that, while they appear black, are just white with a black wax coating. To preserve items that might not be the best of quality, place them in areas where they will not be regularly handled (for example, hang a wreath on your wall instead of on the back of a door which will regularly hit the wall and cause it to crumple to pieces.)
4. Price - This is the biggie. While Goths are no stranger to paying mark-ups for our clothing (that The Cure t-shirt is how much?!), there's no reason this has to be the case when shopping for decor. After all, part of the reason we're shopping at these stores is because it's cheaper and more convenient than buying specialty Goth items. Still, because Halloween items are only available for a short time, stores that sell these items are looking to make as much of a profit off of them as quickly as possible, so the items can often not be worth the full price. If you can, find coupons for chain stores and shop in the after-season sales (be quick, they move into Christmas gear faster than you can say "Bauhaus!") to avoid paying huge mark-ups.
Here is a small list of items which are generally safe to buy during the Halloween sales and which might turn up useful in a Goth's decor:
- Black, red, orange, and purple candles
- Halloween-motif tableware (plastic or ceramic, not paper, please, and note that these might not be microwave safe!)
- Lenticular photographs
- Halloween Flags
- Spooky wreathes
- String lights in unusual colors (purple, red, orange, etc.)
- Candelabra and other candle holders (aim for the metal ones, not paper)
- Black or "bloody" white roses
- Decorated storage jars and containers
- Window clings
- Spooky stuffed animals
- Bolts of black cheesecloth, gauze, etc.
- Halloween knick-knacks (busts of creepy creatures, tombstones, etc.)
- Wall clings
- Paint-able wooden shapes (you can see Bane of GIY: Goth it Yourself putting these to use on an amazing chest of drawers here!)
- Wall posters and signs
I'm afraid I am not overly familiar with your shopping options in other countries, but perhaps someone in the comments would like to lend a hand? For the rest of you, what guidelines do you try to adhere to as far as using Halloween decor year-round? Can you add more to the list of "Safe bets" I have above?