Thatās such a hard question, if only I had a crystal ball! I think things out of fashion at the moment (mahogany, low-mid end art) are at a low point and will come back in the future.
Iād steer clear of any mass produced however ācollectibleā it is marketed as and be wary of ālimited editionā items - thatās o my relevant if more people want to buy the items than the number produced. A limited edition of 500 is meaningless if o my 100 people want to buy them!
Make sure anything you collect isnāt just fashionable right now as that may not last and preferably has intrinsic value. Iām wary of Jellycat toys for instance as although they nice, they are hugely overpriced and fashionable now, but if they go out of fashion (remember Beanie Babies!) they are just reasonable quality stuffed toys, not the quality of Steiff example, which can be similar prices!
Look for quality, quirky, unique items.
Retro gaming is hugely popular, especially Nintendo and every Nintendo console has become collectible, so I think those DS and Wii games that are worth little at the moment are worth hanging on to. And if you can, keep boxes for toys and pop them in the loft. Worst case l, you children might like them in the future for their children, but they might go up in value.
Uranium glass is definitely on the up and can often be picked up in charity shops and at boot sales. You can buy a cheap UV torch online (the USB rechargeable ones are good) to check for it.
There are still a lot of bargains to be found in general auctions, especially in mixed lots, and things do get missed. You can buy lots at auction for just a few pounds which can be sold for profit at Boot Sales and on eBay.