First of all congrats!
I would say if you have been together this long, especially if you live together, it should be a piece of cake really. Of course there will be the few days/weeks where you will be like "Oh my God, we are MARRIED!!" and that will be really cool...but don't expect to get that piece of paper and suddenly everything changes. What you see is pretty much what you get, but that can be a great thing if you are happy and content in your relationship already.
Of course there are the financial aspects, whats yours is his (or hers?) and vice versa and all that, but you already know that.
I would just say compromise, respect, valuing eachother's thoughts, ideas, opinions, putting yourselves and your children above all others -- even other family members when it comes down to it is important too.
I think it basically comes down to working as a team in all things and knowing that you are only as strong as the weakest link so to speak.
Another poster put it well when they said that marriage is 50/50 but not all the time. Sometimes I carry more of the *weight* so to speak, sometimes hubby does, but the important part is at the end of the day it is a true partnership. I am not the boss of him, he is not the boss of me, we walk together...sometimes I lead when he stumbles, or when he wants/needs me too -- sometimes he takes the lead in situations where he is more knowledgable/capable/comfortable.
We try to get everything out on the table when we are upset. Nothing annoys me more or causing more resentment within a marriage (imo) than keeping things inside that upset you. That having been said though, we always attempt to be gentle with eachother and understanding and always keep in mind that words can be very powerful ...so we make it a point not to yell, or belittle, or hurt the other when discussing issues (though no one is perfect of course!)
Trust is paramount in my opinion. Not just trust that my husband won't cheat (which is important too) but trust that he has mine and our daughter's best interest at heart. Trust that I can depend on him to be on time, or to follow through on a promise, or to make decisions in everyone's best interest -- and knowing that he can expect and recieve that same trust on my end.
Good luck to you!
Oh yeah and one thing I read a long time ago that hubby and I do every single day, rain or shine, no matter what is going on....
We give eachother a 30 second hug and a 10 second kiss (or little kisses that add up to 10 seconds lol)....
Of course we touch and cuddle and all that and have sex etc... but on those days where things are hectic, or time is short, or whatever... we ALWAYS at least do the 30 second hug and 10 second kiss.
It works, I'm tellin ya.