If all else fails, use duct tape. It won't look pretty, but you've repaired the crack so it shouldn't leak.
How to Repair a Cracked Coffee Cup. It's time for your early morning coffee, but you're noticing that your coffee cup is sending small drips of coffee down your leg or that your cup holder is full of coffee. You inspect the mug and find a tiny hairline crack that's allowing the coffee to seep out. Well, don't throw that coffee cup out. Try to fix it.
Apply super glue to the crack. You'll need a heavy amount to keep the mug from leaking. You should also make sure the super glue you use is non-toxic. You can try sanding down excess super glue, but you may make the crack worse with the pressure of sanding.
Put the coffee cup into a saucepan and cover with milk. Bring to a boil, then let simmer for a few minutes. Rinse and reuse. Use caution with this tip so the coffee cup doesn't suddenly explode during the heating process.
Use PVA (polyvinyl acetate), also called white glue, to repair the crack. Put just enough to cover the crack completely. If the cup is completely cracked, hold the two pieces in place for a few minutes. Gently scrape away any excess PVA with a sander or an X-Acto knife. However, the white glue will slowly degrade and your mug will leak again, especially if you wash the mug in a dishwasher.