You didn't say how old your dogs are, so I will assume they are adults. Something you can do (no matter the age) is take the dogs out several times a day ON leash. That way, as soon as they #1 or #2, you can reward them with a small treat for "going" outside. You have to reward them immediately though, or they won't know what they are being rewarded for. Do this enough times, and they will realize that going outside is a good thing to do. Eventually you can wean off the treat completely, but remember to continue praising them even after.
I'm a big supporter of scheduled feedings. Not only does it keep the dog from grazing all day long as he/she would if they were free fed(food out all the time), but you have a better idea of when they need to "go". Take/let them out as soon as they've finished eating. Most dogs (it seems especially true in small dogs) go very soon after eating.
I've heard that the rule for how long they can hold it in is to take their age and add 1. That's an estimate of how many hours they can hold their bladder. If they are going during the night, you can give them some water before bedtime, get them to go out and potty and then pick up the water bowl until morning. Some dogs will drink and drink at nighttime when you are sleeping and mess up before you wake.
The best thing to do is to be able to watch them almost constantly for a few days to get a feel for their signals that they need to go and how often they usually need to go. I know most times this isn't possible, but even if you can take a day or so on a weekend when you don't have any plans and just be close by so that you can watch over them, it will help you get a feel for their internal schedules.
Hope this helps.
Oh I meant to add that doggie doors are great for when you aren't home or you are busy with something, but they don't help your dog learn housetraining. If for some reason you were to have to move to a place where you couldn't have one(ie: apartments), you would have to start the training process from scratch. Training at younger ages is easier than training older dogs. IMO doggie doors are a great asset AFTER a dog is already housetrained and knows what is expected of them.