Both people have to want to keep it alive.
One person will never convince another to put in more effort if they're heart is not into it. (At least not for very long)
Being spontaneous is very important as well as never stop dating, sharing wet kisses, holding hands/touching outside of bed, Take showers together from time to time and make time for weekend getaways or vacations a few times a year.
If need be keep a secret "lovers calendar" to track your individual efforts. If you notice it's been a while since you've done something special then do something even if it's leaving a love note in their car, a naughty voice mail, or giving a foot massage after work.
The best way to keep passion alive is to never let it die!
"It's easier to maintain a fire than it is to reignite a spark!"