We don't have a specific formula, unfortunately. But why don't you try these --

 

1. Leave books / pamphlets/ or print out of articles pertaining to salah and its importance, punishment for those who do not offer salah -- over most general places at home, where she would sit/relax. See that you don't be in that area, when she's reading.

 

2. It may happen that she may not listen you or value you much, [It happens that they become more stubborn, when we keep reminding them} , so stop reminding her for few days. But see that you are very good to her.

 

3. Why don't you try out thru her freind, who's religious, and may be her words and discussions with her affect her

 

4. If you know any friend of hers, start praising her. Don't compare them while you speak. This would be a blunder. Just appreciate that friend/cousin for being regular to salah and God-fearing. Insha Allah, it may work.

 

5. If you are a working muslimah, you can seek help of your sister, to get some material, either from net {Islamic sites} or from book stores, about salah and its importance, for your non-muslim friend of your office. Work with hikmah here.

 

6. Problems are part of human life. Moroever a believer is tested many times. Whenever you see your sister in trouble, or depressed with any of her problems, take this as a right oppurtunity to strike. You can softly ask her to seek help from Allah swt thru salah and duas. Explain her how it helps. Insha Allah, this shall bring her closer to Allah swt.

 

Before practicing any of these, never forget to seek help from Allah swt to give hidayah to her, thru any of these.

 

Your job is to give the Message, not bring about the action. Even Prophet Muhammad was reminded in the Qur'an, when he would get worried about his people, that his job was just to spread the Message. I cannot remember the surah or ayat, sorry.

 

Sometimes nagging can also have adverse effects. Best way is to set a good example yourself.

 

Other than that, you can also make dua for her. Pray for her hidayat and strength of Imaan. Make sure your behaviour is that of a good muslim - work on your own Imaan and deeds. Inshallah it will have a positive effect on her. A good muslim is like perfume - those who leave his/her presence leave smelling good... i.e. a good muslim will always have a positive effect on others around him/her, without having to even say a word.

 

And dont get fed up sis - patience and Sabr makes a muslim's character beautiful.