First decide if you just want it to look nice, or if you want it to work nice. For the first there are already lots of suggestions here.

For the second, do you have some users or potential users (right needs, skills, etc.) nearby? Get them to use your software and watch them. Or use screen sharing to do it remotely, along with an audio link. Get them to talk through what they're doing. Watch to see the trouble spots, ways they approach using parts of the software that maybe don't match how you expect people to try to use it. Does the way your user interface is organized help or get in the way? Do the cool widgets you used make sense to people other than you?

Start this early, with the crappiest thing you can get up and running visually. Don't wait until you've invested huge amounts of effort in making things look just so. This may lead you to deeper changes but ones that might make the system a whole lot better.

Or, you could wait until you've put in so much effort and baked in so many decisions about how the system should work that you're not about to make any structural changes. Just put some lipstick on the pig.