Step 1 - read the news
Step 2 - try "Search Neopets" with one word in the item you are looking for. Finding an item there will give you the exact name, type and rarity. And the source code gives the description and the url for the picture. That just leaves the ID#.
Step 3 - use the Shop Wizard. A LOT. If you want to find similar items, don't be too specific in your search (i.e. look for "cracker" not "deluxe tonu cracker")
When you enter a shop, look for other new items that the News may not have mentioned. If an item is in many shops, I usually check the shops that have it priced the highest. They are more likely to belong to collectors and may have other items from the same release.

There are lots of other things you can do, but these three steps will locate about two-thirds of the releases. (Mostly I rely on pootlecat's lists for the remaining third.)