1) Educate yourself.
2) Do not mention the PMS.
3) Cook for them.
4) Get them chocolates.
5) Do not touch them unnecessarily.
6) Do not judge them for crying.
7) Help them with any sort of work.
8) Don't be embarrassed to buy their pads and tampons.
9) Do not breath loudly.
10) Spend time with her.
11) Do not make silly mistakes.
12) Give them a massage.
13) Turn on your romantic side.
14) Watch movies with her.
15) Do not ask for $ex.
16) prepare their hot water bags.
17) Be a listener.
Source: http://www.wittyfeed.com/story/4122/18-Things-You-Must-Do-To-Make-A-Woman-Feel-Better-During-Her-Period