It's an important question.
The biggest factors in rental yield are, of course, location and number of bedrooms. I haven't found that I can add tremendous rental value by making cosmetic changes.
Instead, my focus is on reducing cost ie making improvements to likely problem areas.
Re-roofing, re-stumping and fixing plumbing/electrical problems are all boring, but the potential costs in terms of money and time over the course of the rental period can be significant.
Beyond that, airconditioning adds value, and cosmetic improvements to bathroom and kitchens help attract a few dollars more.