My best advice is that whomever you are marrying that they:
1) are your bestfriend, meaning they are the one you can confide anything in without a shadow of a doubt that they will always support you and never use it against you or turn on you.
2) that you communicate well together.
3) you have a plan.... Money isn't always the biggest problem but it can be. Even if you are broke, if you have a plan its better than flying by the seat of your pants.
4) Make sure you both are planning on going on the same path permanently...I find most of the time the couples are on the same wavelength when they are first together but they fail to accept the differences which initially can bring a wedge between them later.... It can make a difference in the end. MAKE SURE YOU BOTH KNOW WHAT YOU WANT...don't guess that its okay for now.
5) Tell yourself that this is it ...FOR LIFE....If you think its okay if it doesnt work out down the line more than likely you won't try hard enough to make it work *that goes for both of you! Not just one... Taking marriage seriously is a vital step too.