Thinking of you cards can be sent for a variety of occasions and reasons. What you write in the card should reflect the reason for sending and the person to whom you're sending it. There's no clear-cut etiquette or guidelines when it comes to these cards; a lot of it depends on your personal relationship with the card recipient.
Categories & Situations
You may send a thinking of you card for a specific reason or simply to let a person know they're on your mind. The content of a card for someone who is going through a rough time, is sick, had a work setback or a death in the family would vary greatly from cards sent as a surprise or to say "hello." Whether you make the card yourself or buy it in the store, the card should be appropriate to its purpose. To buy a card, go to the card section of your local store. There you'll find rows of "Thinking of You" cards (also filed under "Just Because") with different sayings and images, and even some that are blank. Thinking of you cards are meant to be personal, so keep the recipient in mind while browsing the aisle.
What to Include in Your Card
No matter what card you decide to send, it's what you write in it that's most important. The message should be personal and not generic. Even if you buy a card that has writing in it, it's important to do more than just sign your name and send it. For example, if the person needs some cheering up, write a little joke or list a funny memory the two of you shared. If the person is going through a hardship, let them know that you're there for them and offer support. If you're still stuck on what to write, keep a piece of scrap paper nearby and jot down ideas as they come to mind.
You can always include a little something extra in a thinking of you card. If you like to write poetry, pen a poem to express your feelings. You can also find a poem online and copy or attach it to the card. If you're not good with words, include a picture that expresses your sentiments. Decorate the card with stickers or include something fun like a scratch-off lottery ticket or prayer card (if appropriate) to make an extra special touch.