I’ve tried plenty of things to this point and many of them were failures so I’ll share them with you lest you repeat my mistakes.
Liquor selection – Avoid 190 proof grain alcohol, it can work if you cut it with vodka, but then what’s the point?
Filtration – To speed the process, I’ve made unfiltered batches. No one needs Limoncello that badly, filter it.
Not letting it rest – It’s really hard to be patient, especially with your first few batches, but exercise as much patience as you can. It’s possible to make a batch of Limoncello start to finish in 10 days, but again, no one needs Limoncello that badly. The faster you make it, the worse it will turn out regardless of the ingredients.
Adding ingredients – My only real run in with this was taking a recipe that called for vanilla bean to make the Limoncello smoother. The result was a liquor that tasted like vanilla. It wasn’t bad, but it wasn’t Limoncello either. Using less might make this better but vanilla is a potent flavor.