The bathroom mirror is a great place to write love notes to your wife.  If you share a bathroom with your kids be sure to write notes to them as well.  Just buy a box of dry erase markers (use the ones designed for glass they work better).

I only change out the notes ever 4-6 weeks.  I have found for my wife it continues to pay dividends this long, and that way I don't run out of cute ideas.  Also, start small and simple so that you have somewhere to build from... if it gets results keep going if not you can always quit after a few months.

*Warning:  If you have kids keep it clean.  After a few months of leaving my wife notes on the mirror I branched out to stick figures, and a couple months later I drew a picture of 2 stick figures having sex (yes it is good and ok for married couples to have sex).  The problem came when my college age kids came home one weekend and saw the picture on our bathroom mirror...  We thought it was funny, but it embarrassed the heck out of our daughter.  So don't write or draw anything you don't want others to see (you never know when you might have unexpected house guests).