1.

Use gradients. It adds a cool "3D" effect. But if you don't use it properly it will not look good.

2.

Use colors the right way. Add some saturation for it to look better

3.

Use your keyboard for better shapes. Hold down shift when making a shape for it to be perfect. There are much more but there are already lots of good tutorials.

4.

Be creative. Do everything the way you want. Following these tips and being creative will make your models look 10x better!