Ive been with my Mrs for 27 years Rayna and I still feel unqualified to give advice on this subject..Keith and I are just in our 2.5 years of marriage (almost 4 years of being in a relationship), so I'm not in the right position to give advice yet on how to keep the relationship going for a long time.
About the only thing I could say of any use is that you both have to continue liking each other..
Apart from that...I`m stumped!