How does a husband and wife keep the relationship from becoming stale?

I know communication is key in keeping a marriage together. The day in and day out issues that life brings everyone seems to tear down relationships even if the couple love each other. My husband comes home tired and he isn't a big talker anyway, but I seem to be talking to a wall. We still have fun at times, but he just doesn't seem to feel the same way about me. If I bring it up to him he say he is just tired, etc. I don't want anyone saying that he is having an affair because my husband is not like that. We are both in our 40s and this isn't our first rodeo. We vowed to keep the marriage alive and I think we do a pretty good job, but lately I don't know what is wrong. Maybe it's just growing pains. Like I said he has always been quiet and hasn't ever been a charmer or anything so it's not that he has really changed, but I just don't think he's as happy as he once was.