I have some Transformers from 25 years ago and they didn't really survive that well, but that has a lot to do with the fact that I was a child. What are the best ways to preserve Transformer toys? Currently I keep them on a shelve in my room. Should I keep them in a glass case or keep them in boxes all the time? Are there certain temperatures that are bad for them?

 

IMO there is no way to "preserve" them other than just keeping them out of intense sunlight to prevent yellowing. Like you said it was probably just a side effect of being a kid. They aren't gonna spontaneously combust or anything.

You might wanna check this thread though.

PSA: New toy plastic condition discovered (melted soft rubbery parts)

 

Out of the sun, cool area, that's about it really.

I've been collecting for about 45 years, and white figures are the worst as I've had some go yellow even in their original box in a container (so out of sun) and in cool area, simply because the plastic reacted with oxygen over time.

I've got many that were displayed that are 100% fine, and others that the plastic just got brittle with time... it really comes down to the material and the quality what it's being made.

I even have metal figures/ships/parts that have oxidized, regardless of being Stainless Steel, diecast, chromed, or painted.

The best thing is to keep them clean, or at least be gentle when cleaning. I just use a very soft large paintbrush to dust mine, and if they get sticky from just the stuff in the air over time, a gentle wash with a damp microfiber or soft paper towel, and just a tiny bit of dish soap, but rinsed right away with another cloth/towel and dried very well.