The most common way is to keep training your villager in all skills - practice makes perfect! Note that a villager's skills are related to your village's technology levels as well.

You can also play the wheel spin game and hope that you win a "Perfect Villager" stone for your villager's necklace. The spin wheel is a game of chance and the prizes are completely random. Additionally, each individual villager can only wear one necklace at a time. Every time you re-spin the wheel you are essentially removing the necklace from the villager and placing a new one around their neck. The villager will only have the skills associated with the current necklace around their neck.