Buttons are a familiar UX tool and an essential part of the users' journey. Visual cues help people determine whether or not a button is clickable, so make sure to use proper visual signifiers on clickable elements so they look and function like buttons. Becoming a super hero is a fairly straight forward process:
Fitts Law is a UX principle that says you should make buttons big enough that they’re easy to click/tap and keep the distance short between the action and the action button.
A good rule of thumb from MIT Touch Labs: make your buttons a minimum of 10mm x 10mm. Make buttons mobile responsive and resize with percentage widths.
The primary action on a page carries a stronger visual weight and should be the visually dominant button. Secondary actions (like Cancel or Go Back) should have less visual attraction. A button is multi-state, therefore, consider the hover/tap states and active states of the button.
The effective button style is square with no corner radius or has rounded corners, depending on the site/app brand. Maintain consistency across controls so your user recognizes button elements.
Use standard UI patterns because conventional button placement improves discoverability. Using a standard layout will help users understand the purpose of each element - even if there is a button without other strong visual signifiers. Combining a standard layout with clean visual design and ample whitespace is better design.
UX button microscopy tells users what action will happen if they click the button. For effective calls-to-action, minimize word count: Use "Add to Cart" or "Submit" to make things easy for users.