To GZ: I'm using a fuel system revitalizer, a kind of oil stabilizer. This morning the starting process is still a hard starts. After getting it starts on Saturday, I've rode it for ~20 miles.
http://www.amazon.com/Start-Your-Engine ... B001B0TCN4
To water warrior: Thanks for your suggestion. I have a battery tender but I live in an apartment. I need to take out the battery from my bike and charge it at home, 5 mins dismount the battery at night and 5 mins to mount it in the morning, not a big deal.
I will try to charge my battery ~2 times a week and ride it every sunny day to see what happens.