It is amazing how often the cause of a low quality image is nothing more than a dirty lens. The simple act of cleaning off your lens can help immensely. Clean it often, but only clean it when you must. When you do clean it, be sure to clean both sides of the filter and both sides of the lens, so both the front and back elements as well. Even when you have a UV filter on your lens over time the inside will still get dirty as it is not 100% sealed.
How do you clean a lens? There are many products out there for this purpose but the simplest thing is to simply breathe on the lens and then to wipe it clean with a CLEAN lens cleaning tissue or a microfiber cloth. Microfiber is reusable, however, it gets dirty over time so be sure to wash it very regularly. When it sits at the bottom of your bag it is collecting all kinds of dirt and grit which can scratch a lens permanently. On the other hand, lens paper is meant to be single use and therefore should always be cleaner, but it too it tends to live at the bottom of camera bags where it attracts all kinds of dust. Be sure to keep your packet of lens paper and microfiber in a sealed ziploc bag. Lens tissue is the preferred product for cleaning your lens because it is meant to be used only once and is likely to be more clean.
One last piece of advice, never use the product below. Why? Because once it is used, you can not clean the surface again. https://www.amazon.com/LS-Photography-Cleaning-Circular-Filters/dp/B0197DDXK4/ref=sr_1_5?ie=UTF8&qid=1488397140&sr=8-5&keywords=camera+cleaning+pen