If you look up "P.S." on nciku or do a Google search for "P.S. in Chinese", it's not immediately clear how to actually write a P.S. note at the end of a letter in Chinese.
It's pretty simple, and there are two ways to do it. Note that in Chinese letters, you can stick the P.S. anywhere on the page, although the lower-left corner is normal as in English letters.
The quickest way is just to write 另：and then your extra message.
另：我很想念你。 (lìng: wǒ hěn xiǎngniàn nǐ.): "P.S. I really miss you."
另：我爱你！ (lìng: wǒ ài nǐ!): "P.S. I love you!"
又及 (yòu jí)
Alternatively, you can use 又及. Brownie points to MDBG for having this listed under "P.S.". Nciku hints at it in an example sentence but doesn't give a full explanation.
又及：还有一件事… (yòu jí: hái yǒu yí jiàn shì…): "P.S. One more thing…"
Here are a few Chinese words for postscripts themselves, such as:
附言 (fùyán): postscript (at the end of a letter)
后记 (hòujì): postscript; epilogue
附录 (fùlù): appendix
These words are more usually used for describing postscripts though, not for introducing them.