I've found that incorporating at least one diagonal track part makes most designs look better, particularly with larger coasters.

I also avoid any unrealistically tight twists or bank angle/slope changes.

Edit the terrain around tunnel entrances so as little of the tunnel frame is visible above ground as possible because they look ugly unless they are flush to the ground or a cliff.

It can also help to make a design more "balanced" if possible. What I mean by that is if there is an entire end of a coaster that is high above the ground and away from the rest of the coaster, it might not look as good as if you design it so another lower section of track is underneath that end so there is more going on there (unless you spend time on decorations to counter this).

Building so that the tracks often pass over/under each other also often helps.

Avoid building curved lift hills like the plague unless they are hidden by scenery.

You can build an overbanked turn by building a straight piece of track that slopes up at 45° with a bank angle of 45° in the direction you wish to build the turn, followed by a turn piece that goes from sloped up to level with the bank angle increasing to slightly greater than 90°, followed by another turn piece in the same direction that goes from level to a 45° slope down with the bank angle decreasing back to 45°.

https://parkitect.fandom.com/wiki/Coaster_Stat_Calculation_Guide