While not part of your pin's main design, the attachment you choose is an important consideration. A pin isn't a pin without a post and an attachment.
Function is the most important factor to keep in mind. How will your pin be used in the real world? Is it designed to we worn on a jacket, hat, or other garment or accessory that will require it to stay affixed despite extensive movement and use? Different attachements provide different levels of strength and durability. How large is your pin? Larger and heavier pins typically call for multiple posts and extra strong attachments.
The look of your attachment can provide a nice enchancement or finishing touch to your overall aestehetic. The most common form of visual enchancement is a colored rubber clutch.
Rubber clutches are the most common pin attachment, as they provide the best balance between strength and ease of use. They also allow for customizations in color and even shape.
Rubber clutches are free or inexpensive and are provided by nearly all pin companies. Black and yellow are the default colors that are widely available. Stadri is one of the few pin companies that no only provdes yellow and black clutches for free, but also a dozen others.
Other common attachments are butterfly clutch, spring clutch, and safety pin. See below for more information.