i think that maybe if your sparks fade then you need to step aside and think to your self how much does she/he really mean to you, if she/he were to leave forever how would you feel??? act as if it is the last moment that you two will ever share and maybe that will help.....its all based around love, i have seen couples who have been together untill one passes away and they still had sparks......being apart makes the heart grow fonder most times....
if you do lose the sparks to get them back just do what i said about acting as if it were the last moment you could share,the last hug,the last kiss,last sex, and last breath of words to her/him.....be romantic in love and in love making....be passionate with her/him.......and stuff
BTW what the hell are sparks?