Add more memory to Server?
Try the query below:

SELECT (count(*)*0.0078125) AS 'Pages in Buffer pool'
    ,CASE database_id
        WHEN 32767 THEN 'ResourceDb'
        ELSE db_name(database_id)
        END AS Database_name
FROM sys.dm_os_buffer_descriptors
GROUP BY db_name(database_id) ,database_id
ORDER BY 1 DESC


This will show you the amount of space each DB has occupied in the buffer pool. Note that there are other factors in SQL that also use up the memory apart from the buffer pool.