I always forget to do my special actives on items
I used to always do that, but I came up with a solution when I started smartcasting on everybody last summer:
Think of abilites that relate to the item active, and place them in the slot directly above that key. (Assuming you have default controls.)
For example, when playing leblanc, if I have a DFG, I'll generally put it in slot one, because it has around the same range as Leblanc's Q attack, and I'll always initiate a nuke combo with my Q. (Specifically, Q -> R -> W -> E -> Q if the target is still alive somehow). So I'd basically slide my finger from 1 to Q quickly, since items have no cast time besides tiamat and R. Hydra (and zhonya's, but that's more of a channel than a cast time.), you'd effectively cast the dfg at the same time as your Q. I'd generally have zhonyas above her W attack in slot 2, as I consider zhonyas a type of escape in sticky situations, and her W can be used the same way. If I were say, playing soraka, and I had an m's crucible, I'd put that over her W attack in the second slot, because it's a targeted heal, just as m's crucible, and I'd want to doublecast M's crucible and her heal in that order to benefit the most because of crucible healing based on missing health.
^stuff that became a mini wall of text.