At least when secondhand shopping it’s good to know a while in advance what styles will come. I have had a look and found out what you should be looking for to be ready for next year.

Tayloring will still be hot. If you doubted wether to invest in a good suit, it’s a smart idea to put some money in it now. It’s one of the hottest styles this fall, and since it will last at least another season; go for it!
The 60’s is on it’s way back. Miu Miu already showed some 60’s inspired dresses for last summer, and this fall several 60’s inspired outfits were spotted. So keep it in mind that the 60’s will only come in stronger next year. Get a hold of some nice secondhand 60’s dresses now, and you will be able to wear them for a long time. The headband will also come in stronger, and invest in a short and wide 60’s jacket and nice 60’s accessories.
I will come with more updates as soon as I can, in the mean time, take a look at Sixties City for inspiration.