Here are the 10 categories of successful tshirts on the internet
1. Pop culture recognition humour / copyright violation
2. Self-affirming message
3. Acknowledgement of under-represented group
4. Acknowledgement of under-represented pastime
5. An extremely "clever" image
6. A sensational piece of easily "gettable" wordplay
7. Unexpired meme
8. Food and drink
9. Come-on / invitation to a come-on
10. Juxtaposition/combination of items 1-9
Every time I broke these rules, I ended up with a lot of leftover shirts.
The ultimate non-expiring meme is "cats". You're welcome.