A twice a day brushing should help stop the mattes from forming, and bathing several times a year at least will help also.

Once the mattes are already there, you have little choice but to trim them out. CAUTION, NEVER USE SCISSORS to trim mattes!! Cat skin is very thin and with a mass of hair to deal with it's difficult to not accidentally cut the skin instead of hair. We've stitched up many cats from scissor wounds. Always use electric clippers instead, but still be careful with those as well.