Ditto to what everyone else has said about heath stuff -- hips, elbows, thyroid, heart, etc. Also, make sure you have someone to mentor you throughout this process. If you're not sure when the best time is to mate, there are probably other issues that will come up that you won't be sure how best to handle, either.
If you aren't experienced in breeding, you really need a mentor to walk you through it. As for keeping the lines going... the breeder who bred your dog can do that on his/her own, thus letting you off the hook if you aren't fully into the idea of breeding a dog. It's TON of work and the breeders that I know of rarely every make any money off a litter b/c of what they spent on health stuff for the parents, plus the time and money associated with raising and properly socializing a litter of pups.
It's great that you have homes lined up for the pups. Just make sure you are 100% committed to all that goes into breeding and raising puppies -- including the potential risks of something happening where you lose one or all of the pups, or even the dam herself. Things like that happen way more than we like to think.
As for me, as much as I love my dog and as committed as I am in the dog world (showing in obedience, agility, hunt tests, working trials, dock jumping, therapy dog work, etc.) you couldn't PAY me enough to get involved in breeding! Too much pressure to be responsible for all those little lives!
So, in short, (yeah, right!)... I'm not saying you absolutely shouldn't breed... just saying that if you do, make sure you've got a super-experienced person to walk you through it.
-Stephanie