Couples who shack up before marriage are more likely to divorce, experience domestic violence, have sexual and emotional problems, and be involved in affairs. Yet, regardless of the statistics, people continue to do it.
 
The myth couples use to justify shacking up is that by living together before marriage, they can "test drive the car" and have a more satisfying and longer-lasting marriage. But it's just the opposite.  People shack up because they are skittish about commitment and, therefore, more likely to call it quits when problems arise. 

In addition, couples who shack up actually lose objectivity because they're not looking at the relationship from a distance.  They literally haven't had the "space" to step back and objectively consider whether this person is truly the best match for them.  Instead, they sort of just drift into marriage.

Another reason not to shack up: You won't have a healthy relationship with your extended family.  A supportive extended family is one of the things that makes a marriage work.  However, moms, dads, siblings, and other family members are not going to expend as much effort, caring, and commitment to you as a couple when it's an iffy situation.  People often forget that and then complain about their family not treating their shack-up stud or honey like family. Well hell, if you want them to be treated like family, make them family!
 
Quite frankly, if you shack up, you are basically saying that your future marriage isn't valuable enough to be worth waiting and making tough sacrifices for.  I love it when people shack up and then demand a traditional wedding. How can you choose to live in a tremendously untraditional way and still expect your parents to cough up the money for a traditional party?  If a kid wants to slap the face of tradition, they are on their own. 
 
Finally (and most importantly), shacking up hurts kids.  If (and usually when) a woman gets pregnant in a shack-up situation, there is a high probability that the sperm donor will split within two years, which results in a never-married-single-mom raising a fatherless child. A guy who is screwing a woman without laying down his life for her doesn't want to be a dad - he's just getting off.

In my opinion, the best way to test your compatibility for marriage and reduce your chances of divorce to almost nothing is:

1) Don't have sex until you're married.
2) Date for at least one year before you get engaged.  
3) Participate in a structured premarital counseling program which includes psychological testing. 

However, I know most of you are not going to do that.  So, operate at your own risk - or rather, the risk of your kids.