The best long term solution to cloudy water is to find out what is causing the cloudy water and take care of the problem. In the short term, however, there are water clarifiers available that remove particulate matter which will help the filter clear the pool. These water clarifiers coagulate particles so that they become large enough to fall out of solution and be trapped by the filter. There are many pool owners who use a clarifier on a regular basis to prevent cloudy water. This is especially recommended during the swim season, when cloudy water might keep you from enjoying your pool safely.

The cause of cloudy water is usually either a water chemistry imbalance or poor filtration.
Cloudy water due to water chemistry imbalance

If it is a water chemistry issue, it is usually low sanitizer (chlorine) levels or high pH levels—which makes the sanitizer much less effective. If you have cloudy water and suspect that it is a water chemistry problem, simply test the water and treat as recommended or better yet, take a pint of your pool water to a professional pool store. They will be glad to analyze your pool water and recommend treatment.
Cloudy water due to a poor filtration

Poor filtration can also cause cloudy water. This could be as simple as excessive debris in a pump basket or skimmer basket, poor water circulation due to a dirty filter, or it could possibly be a problem with your filter. There are three common filters used on swimming pools—sand, D.E. (diatomaceous earth), and cartridge. Depending on the type of filter that you have, the following filter problems can occur:

Sand filter – Your sand should be replaced every 3-5 years. If you are unable to clear up cloudy water and have not changed the sand in several years, it is suggested that you change it.
D.E. filter – You could have a torn D.E. filter grid, which allows D.E. to enter the pool, causing cloudy water. Disassemble your filter and check the filter grids for holes or the filter manifold for cracks.
Cartridge filter – You could have a torn or old cartridge—either situation calls for filter
replacement.