That sounds like a very tough situation to be in. Here are some of my thoughts: 
 
- I would make sure and do it face-to-face. 
- I would make sure that they know that you love them and no matter how they feel about your church that you will continue to love them. 
- Be prepared for a big blow up. This will be a big shock to them. No matter what happens you can not reciprocate the animosity. This is when turning the cheek repeatedly is important. You need to let them vent.  
- It's probably going to take a while for them to come to terms with it. But remember to always express your love for them. 
- Never criticize their beliefs or religion. You can share information and positives things about the church, but proseletizing at this point in time probably won't be productive. 
- If you are going to be getting married in the temple you need to tell them way in advance so that they can come to terms with it over an extended period of time. One of the most difficult things will be for them to find out that they will not be able to attend your temple wedding. 
- Always be positive. Always be upbuilding. Never be condescending or arrogant. 
- Attend their church with them on occasion. It will demonstrate that you respect them and their beliefs. 
- Lastly, but most importantly - make sure and approach this episode with prayer and fasting. Ask the Lord for help with providing the right situation at the right time. You want the Lord's help in allowing you to be a member, but to preserve your family relationship.

Dear child of god, if the good lord decides that the best course of action is to not tell your family, then follow his good word and allow yourself to be free of the sorrow.

I have the same problem...exept it's confining my parents that I want to go to the church of LDS. 
 
Thanks for the insightful topic!

As a convert with a father who is near violently against the LDS Church it is difficult to manage honoring God and your parents. Especially when your parents seek to constantly berate and slander your faith.  
 
My best advice is to pray and fast a lot for strength, wisdom, and courage then tell your parents in person that you are a member of the Church of Jesus Christ of Latter-day Saints (things tend to get way more dramatic over the phone or email).  
 
Share your testimony of Joseph Smith, the Book of Mormon, and the Church in a calm and respectful manner. If your parents become hostile then be sure to let them know your faith is firm but at the same time remember to honor thy mother and father (this is very difficult at times!) 
 
Arguments and harsh words will not change anything and while they may judge or even reject you because of your faith know that Christ does not reject you and that he knows your sacrifices in his name! 
 
By living a Christ like life and shoeing through acts that Christ is your Lord and Savior you might even be the catalyst for your parents! (I will be baptizing my mother next month so have faith and stand strong!)