I use on-glaze enamel paints and mix them with oil and turps substitute to get the right consistency. The turps thins the paint down and the oil acts as a binder. I'm currently using lavendar oil, although I'm finding this a bit thin for some purposes, like stippling.
The important thing is to minimize the number of firings. To do this I aim to cover the whole piece with paint for each firing, resisting the tempation to focus on just one area. Each firing typically takes about a minute at 800oC. In this case the enamel lid was quite thin so it only needed 50 seconds.
I recently painted a portrait of Louis Armstrong onto an enamel box lid. I took photos after each firing stage.
Here's after the first firing