The solution is in most cases to sit on your hands and do nothing. This is perfectly normal and desirable. SQL Server reads data into cache and let it stay there, for the simple reason that it faster to access data from RAM than from disk.
If there other applicaitons running on the machine - and that includes SSMS, SSIS, SSRS and SSAS, you may want to limit how much memory SQL Server can get by setting the configuration option "max server memory", tio, say 80% of the total RAM, all depending on what the other applications need. If SQL Server is alone on the machine, you can head for 90%. The less memory you allow for SQL Server, the more disk reads you will get and your performance will degrade.