It depends on a lot of factors, type of plastic, whether the parts need to be strong, can you use a vase print, etc. Here's a few thoughts.
PLA - The brand of PLA makes a big difference, some can be printed very clear, some can't. Most of the transparent PLAs I've used print much more clear at around 240°C.
ABS - I've seen some pretty impressive clear parts printed as a single layer shell in ABS and then vapor smoothed. I tend to find ABS more translucent and less transparent though.
PETG - Again the specific PETG you use matters, but I haven't seen nearly as much variation as with PLA. I'm not sure how much temperature matters, but if it's too hot you get bubbles which will decrease clarity.
Thin Wall Prints - I don't have much experience here, but the Smooth On XTC-3D or vapor smoothing seem to be effective.
If I want transparency, I usually print it at 100% infill (should be a real 100%, too much overextrusion or underextrusion will decrease transparency). Printing slower and with less cooling sometimes helps. It's easier to get the infill solid with a direct drive extruder, I couldn't get decent results with a long bowden tube (a short bowden tube works fine).
Here's a page where I tested 10 transparent filaments, the printed samples are 2mm thick, 100% infill - http://thrinter.com/10-transparent-filaments. Those samples are all overextruded a bit, you can get better results if you dial in the extrusion precisely, but that's hard to get right, and the optimal settings may change slightly depending on the part geometry. Larger nozzles and thicker layers may help to, but I haven't experimented much with that as there are significant downsides to that approach.