I think I collect so many items, I have collections of collections!
I live in earthquake country, so first off, I make sure both I and the collection are not doing to have problems if we get a strong quake. No heavy stuff high up. Really valuable items might not get displayed (being safer in storage), or they will be in a case or securely hung on the wall. Museum wax is great for securing items on shelves and I make sure items on walls have proper hooks and not just a nail.
For standing items, I put them where I can. I have the top of my entertainment center as a display area. My kitchen cabinets have some room at the top, and my more kitschy stuff resides there. (PEZ items just totally brighten up a kitchen!) I've also got some items on the tops of bookcases, both tall and shorter ones.
For multi-sized displays, I tend to mix up my sizes, putting taller and larger items in back and smaller items in front, creating a little community.