When we found out that we were going to have a baby I, like most people, needed to mentally prepare for a huge dip in our discretionary income and thought we'd be living on toast and porridge. This was actually something the hubby and I have been through while trying to survive university. And as fun as those times were, we agreed never to go there again.
One of the benefits of living through times of monetary leanness is that one gets a teensy bit fanatical about budget (spreadsheets, savings graphs, etc), and I've found that the arrival of our Little Man has not really broken the budget.
We've done this just by being conscious of a few things: