My husband and I will celebrate our 20th wedding anniversary at the end of this month. The best preparation we did was to go thru the Gary Smalley video series "Hidden Keys to Loving Relationships" before we wed.
You have to be able to adapt and change with each other. Don't expect that you will be the same people in 5 years that you are now. Life and the living of it produce change; stagnation isn't healthy for water or people/relationships. When we wed, we had zero plans to ever have children; we have seven kids between the ages 15-almost 2 now. Obviously, we changed a great deal over the last two decades. We changed together; I had a change of heart on children a couple years before my husband. We decided *together* that we wanted to have children. The biggest thing holding my husband back was to quite healthy fear of the added responsibility of molding and raising a human being from infancy. We love our kids now, but there is no way that I would have anticipated the life we lead now twenty years ago.
Communicate regularly; don't ever assume that the other person has any idea what you are thinking or feeling. Even if you are *positive* that they know; go ahead and explain anyway just to be sure. Communication goes both ways, so be sure to listen as well, regularly check-up and discuss how you both are doing. Make each other the higher priority, and be sure to only make plans after consulting one another. I have a cool app on my phone that syncs my calendar with my husband's (it also allows us to have a shared grocery list, which is awesome). Do not volun-told your spouse for things; "Sure, my wife/husband can......." On that note; don't volun-told your children for things either. At home you can tell them to do things; when it comes to stuff outside the home (even if you know they will be interested) ask before you commit someone to an activity.
Establish a fixed budget BEFORE marriage that you can agree to. Figure out the high and low levels of budget, formulate a plan and work toward that plan. It is very easy to come to a disagreement over things as stupid as expensive frozen pizza or fast food killing a food budget. Figure out what the basic budget is (including a entertainment budget) and either save the rest or pay off debt. Dream together and form a plan, but don't be surprised when that dream is reshaped. It is far easier to deal with the monetary surprises in life when you have a clear idea of how the monthly and yearly budget looks. Figure out what you can get rid of to save money, and what you each need for health/sanity. Make that budget as specific to the penny as possible. Take an inventory of what you both own, and get rid of duplicates or inferior items. Figure out what you *need* and purchase accordingly via mutal agreement.
Figure out how to split up family holidays (if you need to do that) before marriage. We switched between his parents and mine each year on Thanksgiving/Christmas for a number of years. Also, have certain holidays *just for the two of you* and protect them. Establish a no-phone or internet time on your weekend and just enjoy the company of your spouse. Don't answer your phone, don't look at your phone, and don't check your email. A whole day or the weekend would be great; a couple hours should be a minimum.
Don't ever treat your spouse with less patience than you would your best platonic friend. Often familiarity leads to a lack of basic kindness and respect. On that note; make a rule that neither of you will ever-ever-ever-ever (even in jest) complain about or denigrate the other to friends/family. If you have an issue with one another your FIRST stop should be to discuss it with one another; not mommy, not daddy, and not your friends on or offline. If anyone tries to get you to complain shut them down immediately.
(I am assuming you are celibate, so bear with me on this one) Be patient when it comes to the marriage bed, and be sure to voice your needs, desires, and preferences. The marriage bed absolutely does *not* have to become stagnant, and can actually be more enjoyable as you share it with your spouse. Hurt feelings and resentment can build quickly in this area when spouses don't communicate. Shame can be a driving force that kills enjoyment of that aspect of marriage, and there is no place for it within marriage. On that note; you may find a number of people that believe that a strict marital fast should be adhered to during all fasting periods. Unless your spiritual father specifically instructs you in that regard; don't listen to the hyperdox people in that regard and work something out in a discussion between you and your spouse.
Lastly, and most importantly; when it comes to parenthood the best gift you can give your child is to love their mother/father. You can't shove the importance of the relationship with your spouse down a couple rungs when a child arrives. That relationship can't just be picked up and back to normal months/years later. Sure, for a time after birth it feels very much like your life is surrounding your child. Even in that time it is essential that you take a few moments a day to reconnect with each other. It will be hard, but it is important. In the age of internet and smartphones it is far too easy to replace virtual communication for real communication. There is zero replacement for the in person interaction with your spouse. Text messages, email and such are OK if you can't be together. A handwritten note in a lunchbox, or slipped in a pocket will have a greater effect.
My husband and I came into marriage and parenthood entirely unprepared. We started with being best friends and treating each other as we would like to be treated ourselves. We have endured 2 deployments, buying and selling a first home, buying a second home, moving eight times, the birth of seven children, the miscarriage of 5 children, deaths, earthquakes, floods, near-bankruptcy, conversion to Orthodoxy....and the list goes on and on. Life has had many ups and downs, but we have ridden the rollercoaster together.
I am very happy to be where I am in my life today. Twenty years ago I couldn't have dreamt of being here, but I wouldn't have it any other way.