Newborns can easily be secured in their seat and then lifted into the car, but watch out for your back if you're lifting in older, heavier children. Try a seat like the Bebe Confort Axiss, which rotates in its base and makes things easier.
Make sure straps are not twisted and are at the right height over the shoulders. Pull them tight, but leave enough space so that you can still work your fingers underneath.
Beware of seating children while they're wearing baggy or padded clothing - this can make it difficult to tighten the straps sufficiently.