1. Dont soak it too long
   Dont soak your T-Shirt too long. For the first time, dont soak it more than 30 minutes, and for the next wash no longer than 1 hour. Dont soak it in excessive detergent water, because the detergent will peel the print out.
2. Separate when wash
   Separate your white printed T-Shirt from other clothes (so the print wont be damaged) especially from the color one like black, red, blue, etc. This is for preventing the strong color discolor your white T-Shirt. Also separate the really dirty clothes with the less dirty one so the dirt wont be moved.
3. Dont brush
   Cotton T-Shirt has a soft texture. Avoid brushing this type of shirt because it can impair the texture. Avoid repeatedly rubbing and strong wring because not only can ruin the pore it also can ruin the print.
4. Reversed hang and dry
   Do you know that sun lights can make the color of your shirt and print faded? To avoid it you should hang and dry you shirt reversely.
5. Dont hang
   No, this doesnt mean you cant hang it, but avoid to slip the hanger through the neck, this will make the neck stretched. Also avoid to hang shirt on rope/wire because it can wreck the soft texture.
6. Avoid bleacher
   Dont use bleacher or detergent with bleacher to avoid discoloration and peeled out print. The bleacher also can make the shirt become thinner and rough.
7. Avoid wash with washing machine
   It better to wash your shirt with hand. Using washing machine can make your shirt stretched, and the print will be peeled.
8. Ironing
   It preferable to iron your shirt after washed and dried. Make sure that when you iron the shirt is in really dried condition. It also better to reversed your shirt first so you wont iron the print. Adjust the heat of your iron according to the type of fabric. You can also cover the print with a HVS paper before ironing.
9. Stain
   If your shirt got stained, immediately wash the stained section. Use shampoo or detergent and spread on the stain. Rub softly and rinse with clean water until the stain is gone.
10. Use it wisely
    Use your shirt wisely. Dont use it for sleep. It will be a waste if you use your coolest T-Shirt as replacement of your PJ, rite? Use it for a better occasion.

Good luck with your new T-Shirt :D