Whilst in Southern Australia, it isn't exactly rocket science there are a few tricks that seem to help.
Making sure females are in a reproductive state
With female blotchies, size matters. Age appears to be irrelevant. Personally I've had them breed in their first year (not deliberate), provided that they get at least a month or so of proper brumation (ave temperature at ground level below 13C or thereabouts (55F)).
Generally they will become sexually mature from approximately 40 cm or so under these conditions.
The most important factors are, an adequate brumation period, adequate body size and condition.
By condition I mean body fat! This needs to be on the girl before she goes into brumation. The breakdown of fat in the spring is important in the production of sex hormones which are sensed by males as pheramones. Male blotchies generally show no interest in emaciated females (they don't conform to human stereotypes)

Under natural conditions in the wild many females reproduce every second year with an average of 6 babies with babies typically between 30-50 grams each at birth. This takes a hefty toll on the mothers with babies and associated body products equaling about 1/3 of the gravid weight of the female. Given that babies are often born late - with only a month or so until the brumation period, the females often have little opportunity to put the weight back on.
If breeding is to be attempted in subsequent years, careful management is required particularly if kept outdoors in Southern Australia to ensure that females put on adequate weight.
What does adequate weight look like?
I can't give a recipe such as weight/length ratio etc but with experience you can generally tell an animal that is likely to be successful and one that isn't. In Southern Australia you have to consider that the bluey will generally brumating on or off for 4 to 6 months. That is, living off fat reserves albeit slowly, although in most cases they will make it though in alive and healthy, an underweight one simply won't breed and will use this years a lay year to build up reserves for the subsequent year.
As a generalization females will breed throughout their lives, however it seems to become more difficult to maintain females at a reproductive weight in subsequent years as they get older. A tipping point does appear to be about 7 years of age for many animals that difficulty seems to start in bouncing back post parturition.
It is important to look at breeding females in terms of being blotchies and not attempting to compare with other Tiliqua species for that purpose. Often T.scincoides will still breed even when quite thin.
I've attached a photo below of a female who recently gave birth but had gone off her food and started the brumation process. I've pulled her out and now placed her in a heated enclosure - she has now resumed eating and once she is fat enough (given a week or two's grace to make sure she has no undigested food onboard) I will start the cooling process with her.
Note that she is barrel shaped naturally. She has lines running longitudinally on body which are firm showing little fat reserves.
A close up of the hips shows that there are limited fat reserves where I would want them to be, the space behind the hips should be rounded and plump. Although not clear from the photo, there is obvious bone protrusion in real life.
For comparison here are some photos of Flicker another female who didn't breed this year - whilst not as big as I would want her at the moment, you can see that she has better general plumping with fat reserves and will be ready for action come November.
A close up of her hip area shows it a bit better.