1. Plan for hand-me-downs starting with your first child
First of all, if you suspect you might eventually have more than one child, start planning for it by getting (buying, registering for, inheriting, finding on Craigslist, or making) baby supplies accordingly when you are expecting your first child.
This might mean getting higher-quality items that cost a bit more so they will last through a second or third child, which will cost less in the long run. This is especially true for the big-ticket items like strollers.
It might meaning using cloth diapers, which can typically be reused through at least two children, unlike disposable diapers (that would be gross).
If you are on a tight baby budget, it might be tempting to skip the infant car seat and go straight to a convertible car seat. However, if you plan on having children within a few years of each other, keep in mind that your oldest might still be using the convertible car seat when the second baby comes along, necessitating a second car seat anyway. We decided on an infant car seat (which is also infinitely easier to transport on foot if you are car-free with a baby, the subject of another post altogether) and got a convertible car seat when our oldest was almost 12 months old.
2. Get gender-neutral basics
This is more difficult than it sounds, which you will undoubtedly know if you have walked into a Gymboree lately. Children's clothing manufacturers don't want you to buy gender-neutral clothes that can be reused for second and subsequent babies. They want your girls in princess dresses and your boys in baseball uniforms so you will have to buy a brand-new wardrobe when that opposite-sex sibling comes along. Besides being not a little ridiculous (Do strangers really need to know whether your child is a boy or a girl at one month old? Are you planning on marrying them off anytime soon?), buying a completely gender-specific wardrobe is hardly cost-effective for clothes that will be outgrown after a few weeks anyway.
Basics like onesies, socks, cloth diaper covers, and sometimes sleepers and pants can usually be found in gender-neutral colors or good old-fashioned white. If you're having trouble finding neutral clothes, check the boys' section. Blame it on sexism, but boys' clothes tend to be or at least seem more neutral than girls' clothes. Boys's clothes often include green, gray, yellow, and red, while most girls' clothes go for the pink and purple. It is generally more acceptable for a girl to wear blue than for a boy to wear pink, so between real neutrals and some neutral-ish boys' clothes, you will be set with a wardrobe of basics than can be worn by a son or daughter.
3. Get gender-neutral gear and toys
Your car seat, stroller, swing, bouncer, etc. do not need to be in gender-specific colors, and there are many gender-neutral options for these items. Paying hundreds of dollars for a new crib just so little Reagan can have a pink one is unnecessary. In my opinion, baby carriers and diaper bags belong to the parent's wardrobe, not the baby's, so you can go crazy with the design that matches your own personal style. That being said, if you expect Daddy to carry the baby or the bag, just say "no" to the purple Moby and opt for something more neutral.
Toys, like baby gear, have become surprisingly more gender-specific over the years, as marketers have discovered that parents will pay for a brand-new toy collection if toys are marketed in terms of "his" and hers." For a fascinating look at gender in toy marketing, check out this article on the Center for a New American Dream blog. In the meantime, there are many wonderful toys that are for everyone. Classic toys like blocks, puzzles, wagons, books, and balls, and creative toys like crayons and other art supplies, transcend gender. For big purchases like bicycles and scooters, try to find (again, often marketed as "boys'") red or green items rather than ones with very gender-specific designs or characters. You can even find toy kitchens, dolls, or workbenches that are gender-neutral enough to suggest that either girls or boys can cook, have children, build things, and otherwise engage in human life. By following these toy suggestions, you will also likely avoid the Disneyfied, mass-marketed, and mass-produced toys in favor of handmade or small company's toys (search Etsy for wooden toys, dolls, Waldorf toys and Montessori toys, for a start).
4. Buy or make a few special items
There's no shame in sometimes wanting to dress your baby in gender-specific clothes, and the easiest and most cost-effective way to do this is to get (request, register for, find on Craigslist, or make) a few special items for your baby: a cute dress or two per season for a girl, or an Easter vest or button-down shirt for a boy. These can be paired with neutral items for a feminine or masculine effect, without doing the whole outfit, every day. Trust me, if you put a sweet A-line tunic on your daughter, no one will notice that airplane sleeper underneath (and since when do only boys ride in airplanes, anyway?). Extra points if you stitch or sew your special items for your cherubs - these will be special not only for your little boys and girls, but also for a niece, nephew, a friend's child, or even a grandchild! Now that is what I call an efficient hand-me-down.