I would say the biggest difference between an okay cake and a perfectly smooth cake is the pan coating. If the pan is prepped correctly, then it can be flipped out of the pan right and it will be smooth. Wilton makes a great pan coating that only costs about $3 a bottle. I got tired of running to the store because I ran out, so I found this and have used it religiously!
Equal parts flour, vegetable oil and shortening.
1 cup of each will fill a large cereal bowel and last through several wedding cakes.
I store mine in a tupperware bowel in the fridge. I personally know it lasts for months, but I don't know about longer because I go through it so fast ;).