Hi,
my LO has gone backwards since 3months so I don't feel like an expert but things were good back at 6wks (about 4 hour-long naps a day) so maybe I can help?

my LO was colicky at 6 wks and I found that if I fed him before a nap, he would wake up sooner - my theory was that the digestion and weeing, pooing and burping while sleeping woke him up.
so I stuck to feed-play-sleep instead of feed-sleep. Even though he would occassionally feed to sleep, I found it wasn't worth it because the naps were so short.
I also watched for tired signs like a hawk, he would get tired at about 45-60min after waking up from last sleep.
I used a womb-sounds CD for both day and night sleeps, it really helps I am sure. it is a conditioning tool i think. you could try one.

at 6 wks I would have been thrilled with the night sleeps you are getting. you are doing a great job.

just one more thing... make sure you *keep* putting bubs down to sleep a bit awake... I did this with great results but stopped for just one lousy week and now it's all gone to hell.