When I've done complicated scenes, I've sometimes had to look at certain props to see what texture size they have.  Like what Medzinatar is saying..

Say you have a little table in the background.. Suffice to say you don't need to have a high resolution texture for it unless you have an animation where you move closer to it, or another alternate view that is closer, etc..

Some items, from small clothing, jewelery, and props can use really big textures. Which is nice, for closeup renders.. :)

But what I did was that I made a smaller texture of relevant objects. I checked the objects in the material room, then went into the textures folder and made a scaled down version..  Maybe from 2048 to 512 or even 256 if it was a far away object..  It saved a lot of memory problem. 

Another thing you can do to conserve memory is to check what texture resolution you are viewing with in the preview.. When I map and work with texture, I often max it out, to 2048 or 4096, but when I'm working with a big scene, that's usually not necessary any more so I just pull it down to 512 or so, which conserves a LOT of memory, and minimizes swapping between rendering and preview, too. :)