Acrostacking

Here's this awesome technique I've learnt from modthesims (original link: http://modthesims.info/t/361359) that's called "acrostacking".
I'll leave a small tutorial, pictures included.

Step 1:

Have an object whose height you can't change (you CAN drag up or down a wall most wall lights). In this example I'll be using these curtains which do not reach to the top part of the window.
Hit Ctrl+Shift+C and a cheat box will appear in the top of the screen. Type in "moveobjects on" and hit enter, then open the box again and type in "constrainfloorelevation false". These are NECESSARY for this technique.

Step 2:

Using the "raise terrain" tool, rise the terrain in the area you'd like to place the object. The item will stay still, will not move along the wall's movement. "How much" you should rise the terrin depends how much you want to rise the object.

Step 3:

Grab the objects with the hand tool and place it in the same place it was. It will now snap to the heightened terrain.

Step 4:

Level the terrain. The walls will adjust back, but the object will not come down.
Final result:

Note that if you displace the object, even if you pick it up and leave it in the same place it was, it will come down to its original height. You can still change styles, that will not affect the placement.
You should now disable moveobjects (moveobjects off) and enable constrainfloorelevation (constrainflooelevation true) to prevent game corruption.

Enjoy acrostacking!