Don't leave items or gold lying on the ground any longer than necessary, especially when transferring in town ("muling") as they disappear after a certain amount of time:
10 minutes: Key, Potion, Scroll, Tome, and Ammo (Arrows, Bolts).
10 minutes: Any Armor and Weapon of quality < magic, regardless if flagged as ''Ethereal'', ''Socketed'' or not, and Rune Words.
10 minutes: Pandemonium artifacts (Key of Terror/Hate/Destruction, Mephisto's Brain, Diablo's Horn, Baal's Eye).
10 minutes: Essence (4 flavours) and Token of Absolution (these are new items from Patch 1.13x).
20 minutes: Magic Items, Gems and Runes.
30 minutes: Rare, Set, Unique and Crafted Items.