I just worked overtime like crazy before our first, and we did without whatever we could. So we did not have to go into debt. Then when the federal adoption tax credit trickled after finalization, in we squirreled it away into a separate savings account. Again we did without, and weere very frugal. We also sent a few hundred dollars a month to the "adoption" savings account. We considered it untouchable. It took careful budgeting, and sacrifice, buying most of DD'd stuff second hand - we figured she'd rather have a sibling thatn new stuff!
By the time her baby brother showed up we were okay, and although we used ALL our liquid savings, we did not have to go into debt.
Now with the third... we are about halfway to where we want to be, but hopefully not only halfway through our wait time!!!
We want a baby!!! We'd rather not go into debt - I am a stay-at-home mom and want to stay that way. But we have enough in our home equity line of credit, so we can handle it if it happens.
All I can say is SAVE, SAVE, SAVE! Be frugal, don't eat out, and put the money where you can't touch it. - We did CD's like jaenelle said. Every time we scraped together $500 or $1000. Then it's harder to get out until you need it.
Good luck!