Patterned Skirts, Shorts or Pants
Matching your plain top with this bottom. The patterns will cover your top's plainness and give more attractive look.
Since color block became trend last year, it does not mean that you're old fashioned. Color block will create such an extravagant look and it represents your colorful life.
Necklaces are one of the must have things that you need to keep at home. Besides, they are easy to mix and match especially when you have plain tops. Yet, I prefer wearing long necklaces or bold necklaces instead of wearing small necklaces since they do not attract attention because of their size.
If you are too lazy to be bothered wearing necklaces, you can also use scarves, whether they are no patterns or with patterns. Moreover, you can make your neck warmer in winter :)
If you want to look more formal, you can choose brooches to accessorize your plain top. If your plain top has light material, do not choose a hard-material brooch since it can tear your top.
I hope these ideas will help you to be more fashionable. Remember, being fashionable does not mean that you have to wear latest style clothes. Is is all about you. You can create a stunning look by mixing and matching your clothes.