The Expressions Cheat Sheet

After-Effects comes with a lot of expression controls. These are considered effects and can be found in the Effects menu, although they do not act on your layer in any way.
Instead, you can use each control as a data point in your expressions.

For example, you can connect multiple color properties from multiple layers to one Color Control. That lets you easily change multiple colors at once.
Sliders and checkboxes are also commonly used to easily update expressions without manually changing them.

These controls like regular controls let you set keyframes, which is also a great way to manipulate your expressions over time.

To add new expression controls, go to Effect -> Expression Controls, then choose the desired one you want to add.

In order to reference a control in your expressions, use the Pickwhip like so:

About that Checkbox Control

The checkbox control returns a value which is either 0 or 1. In JavaScript, 0 and 1 are synonymous to false and true, which means you can use them as a condition (read more about truthy and falsy).
However, in my experience Pickwhiping to the Checkbox Control is sometimes not enough to get it working properly and that's most likely due to precision issues.

A walk-around that works for me is to add ".value" after the Pickwhip result, or put the entire thing inside a Math.round() function.

//Option A:
effect("Checkbox Control")("Checkbox").value;
//Option B:
Math.round(effect("Checkbox Control")("Checkbox"));