For the past 2 years, we been using the white gene as our selectable marker because the flies seem to be much healthier than the flies we used to use with a rosy marker. I now prefer using a white mutant stock called yw; *2,3Sb-/TM6Ubx because it expresses the transposase. If you use this line, you also must maintain a separate stock called yw which will be used to remove the transposase gene (*2,3) during the establishment of a transposed line. Alternatively, you can inject yw embryos with a mixture of your transposon plasmid and another plasmid (eg. p13wc) that encodes transposase. The plasmid encoding the transposase is lost as the fly develops so there is no need to take special measures to eliminate the transposase gene. The reason the transposase gene must be eliminated is to prevent the transposon from being mobilized and duplicated in subsequent generations. Both methods work well but I found the *2,3 approach gave a higher efficiency of transformation.
It is important that your injection stock be healthy. Inoculate at least two bottles with approximately 100 adults each. The bottle should contain a monolayer of dry yeast the size of a dime. After about a week, these adults should be moved to fresh bottles. You shouldn't keep the adults in a bottle for more than a week; otherwise, the bottle gets too crowded with larvae. In fact, you may find that a week is too long.
The next generation of adults should begin to eclose in 12 to 14 days. Transfer the adults to the clear cylindrical cages about 3 days after they have begun eclosing. The flies in the cages should be maintained on grape or apple plates. If there are plates available from the large population, you can scrape the material off one of these plates into a beaker. Melt the material in the microwave and pour the material into petri dishes. Once the food hardens, store the dishes in the refrigerator. You can also prepare your own mixture using the autoclave. This is as easy as making bacterial plates and the directions can be found here (you'll probably want halve the preparation). We just started using this procedure to make plates and the large population of flies seems to do well on either grape or apple plates.
You need to change the plates at least once a day. If your simply maintaining the flies, sprinkle dry yeast over the entire plate at a density of a few grains per 0.5 cm2 . Gauge the amount of yeast by whether it is all consumed by the time the plates are changed again.
To prepare for injections, provide the flies with a freshly yeasted plate several hours before you intend to start injections. Change plate every hour. This encourages the flies to lay mature eggs that they have been holding during the night. The flies will probably be ready for injections after about 2 plate changes. You can find details for how to do the dechorianation and injections here.