Boy does that bring back memories. When my second baby was about 3 months old she started waking up in the middle of the night and nothing, and I mean nothing, would put her back to sleep but a bottle. Before it was over, she was up to 3-4 times a night. Doc told DH to check into a motel so I could just let her cry herself back to sleep! It took about 2 nights of that to break the cycle.
So, to summarize, been there, done that, and don't want to do it again!
But also must say, try to enjoy them while they are young as much as you can -- that nest can feel very empty before you know what hit you.