After a while relationships are definitely at risk of loosing that spark. You might start to not feel quite as excited about seeing each other and things can seem to fall into a bit of a routine, especially if you see each other too much - don't let it go into overkill - spend time with each other because you want to, not because you feel you have to.
But the good thing about being with someone for a long time is that you know the other person inside out and you can both become more open with each other, about how you feel and what you want. Try new things, go new places. A relationship shouldn't be based on sex, but since you've become closer you might be more comfortable in telling each other your fantasies and open to trying out new things in the bedroom to add a bit of spice.
Dress up, invest in some toys, treat yourselves to a night in a hotel, but only do things you both feel comfortable with!