Apes are carrying mammals, so carry your baby. Front carriers, back carriers, baby slings.
With my three children I used all the three styles. Front carriers you can use right from the beginning, facing inward or forward, but when a baby grows older and becomes heavier and more alert (starts grabbing your ice cream), back carriers are better. Baby slings are good for a newborn and a toddler alike, but some people find it difficult to distribute the weight in the sling. I used front- and back packs with the firmer support for the long walks and slings at home. Some people use slings even for older children, but our slings were all worn to the threads by the end of the second year. I used them to make the rag dolls.
A manufacturer's instruction usually gives both suitable weight and age for different position. Usually a baby can face outward once he can hold his head (2.5 to 3 month), and use a backpack once he can sit on his own (5.5-7 month).
Front carriers (kangaroo-style) are made of padded fabric and are machine washable. They have soft adjustable shoulder straps and sometimes a padded belt, and snaps or straps to shorten them for a smaller baby. Baby can face backward (toward adult's chest) or outward. When facing outward, watch for baby's arms: he can reach almost as far as you can in front of you and will grab pans and knives given the chance. The suspension is very important: 7-15 extra pounds against one's chest is not the best weight distribution, and with the poor support your back is sure to ache. Models vary in size and some minor details like organizer pockets for diapers or bottles, but look for the hip belt and wide, comfortable straps that cross on your back. I used Evenflo ($50 at Burlington year 2000) front-carrier with my first child and Baby Bjorn ($50-70 online) with two others, and found the latter far superior, even as my model had no hip belt. By the way, it was a hand-me-down, and I handed it further with no visible wear.
There are also side-carriers on the market. They have one wide diagonal strap across the back instead of shoulder straps and allow front, side and back (astride the small of your back, no metal frame) positions. The weight of a baby is on the waist level, which is easier on your back, and they are obviously more flexible than front- and backpacks. I never used one.
Back carriers usually have aluminium frame and a hip belt, like a hiking backpack. They are good for children up to 40 pounds, some models claim 60 (ouch!). Models vary in the frame construction and additional features like cup-, bottle- or diaper holders, detachable pockets that turn in a toddler-sized backpack, canopies and moskito screens. A child seats in a fabric saddle, supported by the back and sides of a carrier, and harnessed with a 5-point system (shoulder belts are often padded) and there are usually "stirrups" for feet. Children like to ride high and can sleep on your back, too, but there is no comfortable support for a sleeping child. Keep in mind that a 6-9 month old baby will get tired sooner than you.
I suggest getting the back carrier in a sports store: the brands they carry there are more likely to have an adequate support. I used Kelty Ridgeline ($160) and the now discontinued Kelty Country ($100). Both have a 5-point harness and a detachable pocket. I still own them both and they are in a great shape after 6 years and 3 children, but the Ridgeline is sturdier and have an adjustable belt position -- it's essentially a better backpack.
There are also backpacks/stroller convertibles. Never had one, never tried. Kelty has one such.
Baby sling is a long (2 to 4 yards) wide (2 to 3 feet) cotton scarf. The longer ones just wrapped around a mother and a baby together and tied. I used the short sling called over-the-shoulder baby holder. It is padded along the sides, and one end is stitched in a thick narrow strip and threaded through the two 2.5" rings on the other end. The quilted shoulder pad is stitched to the sling under the rings. To put the sling on, pull it over the head and one shoulder, leaving the pad on the other and the rings with a threaded strip on the front. Spread the fabric evenly on your back and put the baby in the fold of a sling in front of you. Pull the strip until the sling is comfortably tight. For a newborn, put on the sling backwards and use a shoulder pad to support and cushion baby's head. Put the sleepy baby face in, and alert one face out, feet inside or dangling under the lower rim. You can even keep the 3-5 month old's arms inside the sling for his and yours safety. The older baby or a toddler can ride astride you hip or a small of your back, secured with the sling. You can nurse the baby inside the sling discretely, even as you walk. If you baby fell asleep, you can put him down and crawl out of the sling, leaving it like a thin blanket. Toddlers often associate slings with comfort and carry them around while not carried themselves.
The sling is better for a newborn than the front carrier because it keeps the baby next to you in every comfortable position, and allows him to feel you skin, breath and heartbeat. Since it's just one layer of a thin fabric child will not get too hot, like he could in a frontpack, but you have to readjust the sling often, and it's asymmetric. It's also almost impossible to wear a sling with the bulky outdoors clothes. The sling will not let you walk for hours like a carrier with two shoulder straps will. It's rather an extra arm than a stroller substitute.
Over the shoulder carriers come in one-size-fits-all, but very small or very large parents may want a custom fit. Slings are sold on the web by many vendors ($30-50 is a fair price), most of them are made in the US, and you can make one yourself. Cotton broadcloth or quilting fabric is $4-15/yard depending on brand and pattern, you will need 2 yards and 2 wood, metal or plastic rings. Sling can be a beautiful thing to wear, too.
See also: children, jogging stroller, baby car seat.