Before you're engaged (you can start doing these thing before you even start dating!)
Go to church, regularly.
Get the spiritual basics in everyday (scripture study & prayer)
Learn homemaking skills, cooking cleaning, fixing. This goes for the guys too.
Learn how to manage money & learn to live frugally, even if you don't have to.
Learn to listen and follow the spirit. This is essential for finding your future spouse as well as life after marriage.
Learn to communicate. Duh!
Learn to serve others. If you haven't learned this by the time you get married, it will be harder to learn after. If you haven't learned it by the time you have kids, they'll teach you by force!
Things to do after you're engaged
Continue to do all of the above, personalized toward your spouse
Talk, talk, talk! Talk about everything with your future spouse.
Plan more than the wedding. Plan mostly for the marriage!
What are you doing or did you do to prepare for your marriage?