Using webs without API's

I need to use some web services for automate some tasks. Those webs don't offer any kind of API to talk to, but they have some forms, some of them use login (sessions and the like).

Im trying to access them by surfing the forms with firebug on so I can read what they are doing inside and then sending curl commands and parsing the answers but it's getting really hard.

Am I doing it right? There is other tools I can use? Do you know any tutorial or some indepth resource I can read / study to learn how to do this better (I dont mind if its a book / spending months studying)

THanks for your time

EDIT: I forgot to mention, the regular html are no problem with javascript + php are a bit harder but I manage to get them working. My main problems lay when they use ".cgi" ajax or when they use authentification (i have the credentials to log in but i struggle to send them and pass the cookie/session to the next request).