Before you do anything, make sure that you have a plan. As the saying goes, if you fail to plan, you plan to fail. Try and decide what is the goal of your farm.
Do you want to expand it?
If that's the case, the quickest way I can think of would be by adding neighbors. If your objective is to earn more coins in a short amount of time, focus on purchasing crops that take a shorter time to mature but provide fast rewards.
If you want to find out about the latest features in FarmVille, just keep your coins and wait for the right time. Otherwise, you'll use up all the coins and have nothing to spend on any upcoming features.
Spend Wisely
If you're already set on building a bigger farm and leveling up, don't spend too much coins purchasing decorations or animals because they don't serve much purpose and take up space in your farm. Remember to sell unwanted animals or give them to neighbors who will hopefully return the favor by giving you other items.
Visit Neighbors
There are other ways of earning more experience points such as visiting a neighbor's farm and helping them. Visiting neighbors and helping them with chores will give you 5 experience points so keep adding new neighbors. You can visit them everyday so the more neighbors you have, the better.
Plant the right crops
Part of playing FarmVille and being a good farmer is to plant the proper crops to maximize your profits and earn lots of coins. If you know that you can only play FarmVille for a short amount of time, invest in crops that will take an hour or two to mature.