I initially had issues with prints not sticking properly to the print bed. I tried to solve it with painter’s tape and glue, but then found out that my issues were due to improper height calibration. The first layer was actually much thicker than it was supposed to be, which meant that it was applied with too little pressure.
This post on the Prusa forum helped me. The Prusa i3 does have automatic bed leveling using the PINDA probe, but the absolute height is of course dependent on the height of the PINDA probe above the bed. That’s why there is a “Live adjust Z” function in the menu that lets you compensate for differences between the optimal and actual probe height. I measured my first layer with digital calipers, and it turned out that my first layer was actually around 0,39mm thick instead of 0,2mm. By setting the Live adjust Z to -190um (it’s in micrometers), I got a first layer of the correct height and thus proper adhesion of prints to the bed.
Thanks to the printer’s PEI print bed surface, there is then no need for painter’s tape or glue. Just clean the bed with Isopropyl Alcohol (voor de Nederlanders: bijv. bij ReplaceDirect online verkrijgbaar) on a paper towel. The manual states that PETG may stick a bit too well (this does of course depend on the size of the first layer), using a window cleaner instead is then recommended.
Very small parts may still have issues, the contact surface of the first layer is then simply too small. A brim or raft (which can be enabled in the Slic3r settings) is the way to go in this case.