Capitalization & punctuation
Capitalization

Do.

Don't.
Contractions

Do.

Don't.
Commas
Use the serial comma in a list of 3 or more items. Serial commas add clarity.
Without the serial comma, individual items in your list may be incorrectly heard or read as groups (e.g., “daisies and sunflowers” sound like they come together, while “daisies, and sunflowers” are clearly separate).

Do.

Don't.
Exclamation points

Do.
Use common terminology that is familiar to most people (like "sign up as a member").

Don't.
Numerals

Do.

Don't.
Symbols

Do.

Don't.
Time

Do.

Don't.