@Bittersweet This is why we decided to find out the sex with this baby! My first is a boy (we didn't find out sex), and I've always envisioned having a girl. It took me a good long 4 months to really bond with my firstborn when he was born, and I attributed that partially to the fact that I had to call him "it" or "the baby" while pregnant instead of being able to put a she/he to him, or think seriously about names.
We did find out with this second one, because I knew I'd be disappointed with another boy. Lo and behold, it's another boy. Found out at 21 weeks. I'm just now getting to the point where I can accept my disappointment and move forward. I'm enjoying thinking about him as his possible names, and about having two boys who will cause trouble and romp around together. I think it's really going to help with bonding after birth too (at least I hope so - 4 months was a really long time with our first).
So if you had trouble bonding with your others at birth, maybe think about finding out? If you bonded right away, then you may be just fine---most people talk about forgetting everything else when the baby is born, you're just excited they're here and healthy. It just wasn't the case for me. I felt so alienated from my firstborn. May have been circumstantial given how his birth occurred, but it's definitely something I want to try to avoid with this second one.
@MadamRose makes a good point though---I think it's a smart idea to convince yourself of the sex you don't want, and then if it's the opposite, you'll be even happier! I'm not the type of person that would be able to convince myself like that, though