0 to 6 months
Consider this: Your child will spend the vast amount of his first three months sleeping, eating and pooping. And those last two often result in messy clothes thanks to spit up and, well, leakage (eww). He's your sweet baby, so it's not a big deal -- but you shouldn't waste your money on expensive outfits that he will outgrow or spit up on, or that will be covered up by your baby's swaddle blanket. (Now, that you can splurge on!)
For everyday dressing, make sure Baby has plenty of Onesies and layette sets (longalls, Onesies with matching pants, etc.), gowns for nighttime (which make for easy middle-of-the-night changes), booties and hats to keep her extremities warm. Always dress her in one more layer than you would dress yourself in any weather. She may go through several changes a day due to soils, so stock up on the basics.
Splurge on maybe five nice outfits for your baby to wear when company comes, when you have photos taken, or for special occasions like a christening or holiday.
Mom, remember: This is a big spit-up period, so keep a burp cloth handy to protect your own clothes!
6 to 9 months
Your baby is still growing at a rapid pace, so keep in mind that he will grow out of everything you buy for this stage in just a matter of months. During this period, he may start to go mobile -- rolling, "army crawling," scooting, sliding around on the floor -- so save all the nicer clothes for special outings when your baby won't be on the ground as much. Also, as he starts eating solid foods, stock up on bibs so his clothes don't get ruined during messy mealtimes. Your baby may start crawling at some point during this time. Be careful with dresses for a baby girl, as they often get stuck under her knees as she begins to crawl, causing her to trip. Rompers and playsets are great ideas during this stage.
9 to 12 months
It's time to play! Think comfortable separates such as t-shirts, soft denim, leggings and other attire that allows your baby to move. Make sure the bottoms you buy still have snap closures that make for easy diaper changes. If your baby girl is walking now, pull out those dresses you put away while she was learning to crawl... assuming they still fit.
He really doesn't need shoes at this point, but if you must cover his feet, invest in a good pair of soft-soled shoes such as Pedipeds or Robeez that are designed not to hinder the growth of little feet.
The right fit
Sure, you're tempted to buy clothing a bit big for your fast-growing baby, but resist the temptation. Loose clothing can get caught up on something, or she could simply trip on it. Shoes that are not the right fit are never a good idea, big or small. Always buy well fitting clothes for your baby, and put clothing that is too big away for the future. Send too-small items along to family and friends with babies on the way, or sell or donate them. Don't forget to save a few of baby's outfits (along with that first pair of shoes) to commemorate the first year -- a milestone!
Don't forget other staples such as bath towels and plenty of large blankets for swaddling (in the first four months) as well as for covering the floor on which your baby will lie/crawl during tummy time.
When you leave the house, prepare yourself with an extra change of clothes, blankets, socks and perhaps an extra layer just in case. (Stashing a spare outfit for yourself in the car during this first year isn't a bad idea, either. You never know when a messy incident might occur!)
More ways to prepare for baby: