Obviously start with high quality graphics but from the printer's standpoint, quality is all about dot placement. The more accurate the head can fire a dot, the higher quality you will have.
Environment: Part of this means keeping the printer in an environment that is tailored to the printer's needs. Check out your manual for the recommended temperature and humidity for your printer. If the environment is not good, the head cannot fire dots accurately.
Alignment: Learn how to do the print head alignments yourself. Many people buy printers and never get trained or just completely ignore maintenance which includes being able to do alignments. Your head will slowly lose alignment through the years due to wear and if you never re-align it, quality will suffer. The alignments basically are what directly changes the timing of the signal to the head so the dots are placed accurately. I see signs every day in the public that are obviously out of alignment and I think to myself, "How does anyone run a business like this?"
Print Settings and Media: As long as your machine is in the correct environment and the heads are properly aligned, now you just have to setup your RIP settings for high quality printing. The highest resolution your printer prints is 1440 x 1440. The slower the head moves, the better the quality. Some machines allow you to select the head speed and some don't but slower is always better. Printing in unidirectional will also be higher quality due to the print direction being the same for every pass and therefore no lawnmower effect. Also, the more passes the less likely you are to see defects. A profile with variable dot is also higher quality due to the fact that the head prints different sized dots to achieve smooth gradients and transitions such as skin tons and landscapes. So to recap, 1440 x 1440, slow motor speed, unidirectional, and variable dot will give you the highest quality possible.
That being said, if your machine is aligned properly and you are using high quality vinyl, you should be able to print at 720 x 720, 8 pass, bidirectional and fast motor speed and still achieve high quality prints.
Edit: I forgot to add, yes making your own profile is always superior. Not completely necessary if you can find a good enough pre-made profile but always better to have complete control of your color process.