Here's a hint from self experience:
If your relationship is getting boring after a while, sex is turning to something "you just do"...maybe it's time to spice it up!
Be creative, let your fantasy play. Men are
visual creatures so get some sexy lingerie and whatever else you can come up with =)
The most well known and most expensive:
http://www.victoriassecret.com The offline shop my girlfriend bought before:
http://www.lasenza.com The online shop for the undecided and shy:
http://www.romanceculture.com