There are a few things you can try here, depending on the situation.

If you want to remember to take something with you when you leave for work, the simplest way is often to place that object by the door, in a way that you can’t miss when you walk out. It’s best if the object physically obstructs your path. However, that’s not mandatory. If you can’t do this (e.g. for fear of small kids moving or damaging the object), you can place a secondary memory object in a visible place, to remind you to take the first one.

Sticky notes can also serve as useful ad-hoc reminders. Advanced users of this tool might even purchase some colored plasticine to serve as memory objects, which can be shaped in various ways for different purposes.

Crossing your fingers to remember is as easy as it sounds.  Simply cross two fingers, and keep them crossed until you are done with the conversation. When you get a spare moment and notice that your fingers are crossed, this will trigger you to remember and note down your original thought. If you find that your fingers uncross without you noticing, try tightening up your cross, and your awareness.