I'm in the same boat, henhao. Sometimes she takes a nap for two hours, sometimes only ten minutes. I read in Healthy Sleep Habits, Happy Child that babies the age of ours have no schedule yet. They start producing more melatonin about 3 mos of age, and their nervous system is more organized and it gets a litte more predicatable then (though growth spurt and other "disruptions" still occur).
When my girl gets so tired and won't sleep, I just hold her and rock her while giving her the pacifier (after I've nursed her till she pushes away the boob). Eventually she sleeps. It just depends. I also use a swing sometimes. Or I sling her for hours, but that;s hard on my back. I also sleep with her, which someitmes helps her sleep longer. The best advice on here someone gave me was to try the same thing till she falls asleep rather than a bunch of different tactics, otherwise she gets more stimulated.
I guess we just have to hang in there.