When I'm in a slump I either do "fun rides" with friends (today we just rode from the coffee shop to the bagle store, ride a little, eat a little..) or I stay off the bike and do other sports and activities for awhile (running, swimming, yoga). I think it just gets worse if I try to "force" myself to ride.
I go through peaks and valleys with my riding throughout the year and try not to stress too much over the whole thing.