On Page Dropdowns: Design Guidelines

Dropdown boxes and menus are overused and clunky but can be useful for revealing a list of options or commands.

1. Avoid interacting menus, where the options in one menu change when users select something in another menu on the same page
2. Gray out any unavailable options
3. Very long dropdowns that require scrolling make it impossible for users to see all their choices in one glance
4. Avoid dropdown boxes when typing may be faster
5. Avoid dropdown boxes for data that is highly familiar to your users
6. Keep the menu label or description in view when the dropdown is open
7. Keep global navigation options out of dropdown menus on desktop
8. Support keyboard input to navigate within a dropdown

Source: Dropdowns: Design Guidelines