Hey howdy Hey! ;-) I'll get started on the common Questions before I go do some housework (blah)
~What is the proper way to put my infant into her infant carrier?
** I'm not really sure about this question but I'll try here. What I did was loosened the straps, move them out of the way, remove any puffy clothing (coats, snowsuits etc) put them in buckle, tighten the harness til it's snug as a Hug, chest clip at armpit level. Straps need to be AT or below the shoulders for rear facing.
~My baby is very small, only a little over 5 pounds. I've heard that those inserts that help the baby to fit in the infant carrier aren't the best things to use. Can you tell me why and what I should use instead??? Do you have any pictures so that I can actually see what it is I should do?
** The general rule of thumb is if it didn't come with the car seat it's not safe to use with it. there are some exceptions to that but if the item in question goes between or behind your baby and the harness it's not safe to use and things like strap covers and hard toys are a no no as well. :-)
What you can do to help support a small infant is roll up receiving blankets and put them on either side of the baby. And if there is a gap between the crotch portion of the harness and your baby's groin area you can roll up a wash cloth and put it in that space to help out. I do have pictures... this is my precious baby Stitch ;-) in a Graco SS1. Click to enlarge, the first is baby with just rolled receiving blankets and the second is with the R. Blankets and the rolled washcloth.

~What infant carriers are the best fit for a preemie?
** The Safety 1st Onboard 35 is rated from 4-35#, so it's likely to fit a small infant well.
The Mia Moda Viva doesn't state a lower limit (birth from 22#) and it's a smaller seat so it would likely work pretty well for a small infant.
Another issue that would be encountered with a car seat and a preemie (or even just a small infant) is harness height, a 5# preemie should technically fit in a Snugride but the lowest harness slot may be over their shoulders so they don't fit by height. A Couple infant seats with low bottom slots are the Britax Chaperone (new version of the Companion)-6.25" bottom slots, Baby Trend Flex Loc 7" bottom slots, Graco Safe Seat 1 (now the Snugride 32) - 7" bottom slots, Evenflo Embrace -5" bottom slots & Combi Centre with 7.5" bottom slots.
To figure out where baby is in slot height you measure from butt to shoulder. :-)
~After my child outgrows the infant carrier, what is the best kind of seat to get? I see these "all in one" seats and it seems like a good idea to pay once for all the kinds of seats my kid will need. What are your thoughts on these "all in one" seats?
** A rear facing convertible is typically the best choice for any baby after outgrowing the bucket seat. I don't care for the all in one car seats, they generally do everything OKAY but not great. A couple are okay RFing, OKAY FFing and make an attrocious booster seat. The harness is typically outgrown before the child is mature or big enough for a booster. Another thing is expiration, most car seats are only good for 6yrs from DOM (date of manufactuer). So let's say your baby outgrows the bucket at 8 months and you buy an all in one seat, that seat will be expired by the time your child is between 6- 6 1/2yrs old. I started my oldest in a booster at 6yo so I would have barely gotten to use the booster before having to buy another. So not the only seat I'll ever need. ;-) I think it's better to get a good convertible and then buy a dedicated booster when it's time. You don't really save any money with the all in one seats anyway.
~I have heard alot of people talking about extended rear-facing. What is that and why is it a good thing to think about and do?
** That's a long one. ;-) to get all of that info I have it in one neat pretty place that can be read here: http://messageboards.ivillage.com/n/mb/message.asp?webtag=iv-ppcarseats&msg=69595.1
I'll copy important points from that thread but the info and some great videos are in there.
According to NHTSA, a rear-facing car seat is 71 percent safer than no restraint at all, and a forward-facing car seat is 54 percent safer than no restraint at all.
When a child is forward-facing, there is a lot of stress put on his/her neck in a crash. The weight of a child's head in a crash causes the spinal column to stretch...the spinal cord, however, is NOT meant to stretch! This can cause a tear...which means paralysis or even death. This is referred to as "internal decapitation"...the child's head would be slumped forward and it would look as though he/she was sleeping. It doesn't matter if the child has great head control...that means nothing. New data is showing that a forward-facing child is 4 times more likely to be seriously injured or killed than a rear-facing child of the same age.
Rear-facing seats do such a great job of protecting children because the back of the carseat absorbs the crash forces. The child's head, neck, and spine are kept in alignment, allowing the carseat to absorb the forces. The child's head is also kept contained in the carseat, decreasing the risk of coming into contact with projectiles.
What about big babies? A 95th percentile baby may look stronger than his 5th percentile friend, but in a crash the bigger baby is likely MORE at risk if he's riding forward-facing. The rigidity of bones and the strength of ligaments in the spine is likely the same in children of the same age, no matter their size. And a 95th percentile baby likely has a much larger, heavier head, which will pull forward which much more force than that of a 5th percentile child.