IMO 12weeks is too young for sleep training. My ds only ever woke up one time in the night, and I give Gina Ford CLBB the credit for that. I would recommend getting him on a routine so you can be sure that he is getting the right amount to drink (and sleep) at the most advantageous intervals for sleeping through, or at least only waking once. When he does wake the first time I would make sure that you give him both boobs even if you have to rouse him to take the second.
I didn't follow GF's recommendations about expressing, but if I had my time again I definitely would, to make sure that I had enough milk to get him to sleep at 7pm from an earlier age.
I was trying to make it to 6mths before initiating the controlled crying but ended up doing it at 5mths with good results, although it is easy for them to slip back into old habits if there routine is disrupted by sickness or in my case having visitors I didn't want to wake.
I had sleep training instructions from a midwife, but I also read a great post on a US website from an Australian midwife, that was less reliant on the whole timing thing and more on analysing the crying i.e., were they getting worked up or winding down, were there gaps between cries etc and she also said you should follow your instincts on whether to pick up or not. The longest I let ds cry when in training was 10mins before going in. I have also stopped having a nite lite in his room from early on and only operate with the light from the hall through a slightly open door.
As recommended by Aloha on another thread today, I used Mumsnet to distract me during training, as if I had laid in bed or stood outside his door I would have gone mad.
HTH