yeah, I think newborns and younger babies are so much easier when it comes to sleep! you wrap them up nice and snug, lay them down, and they sleep (at least many of them do). but then when they learn new skills like sitting, standing, walking...sleep gets much worse.
i'm not sure i have the advice you want, but i rock my baby any time he/she won't fall asleep without crying. i'm not a CIO type of parent, so that might not fit with what you're comfortable with. my current baby was my best sleeper of them all. she'd snuggle in her blanket and dose off to sleep. it didn't really require any sleep training. but then she began to sit and it didn't work anymore. So, i rock her and she goes to sleep. that's still what I do. i enjoy the snuggle time. i've done this with my other three and haven't regretted it:-) though there are those days i'd wished they would just lay down and sleep! but i don't like to let them cry. and they aren't babies for very long...it's a time to treasure!
that all being said, i do think they learn to adjust to their new skills and would probably stop finding sitting to be so exciting. but likely the change will be that they learn to play for a little while and then fall asleep.