physical memory usage is high on production sql server

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http://serverfault.com – I'm going threw high physical memory usage in SQL Server. It always around 90%.When i reboot the server, it will initially good but end of the day it will around 90to 95% physical memory usage. Configurataion : Windows SQL server 2008r2 Processor Intel Xeon CPU E5450 @3.00GHZ Installed memory 32.GB System type: 64bit operating system. I've installed Microsoft SQL Server 2008 (SP2) - 10.0.4000.0 (X64) Sep 16 2010 19:43:16 Copyright (c) 1988-2008 Microsoft Corporation Enterprise Edition (64-bit) on Windows NT 6.1 (Build 7600: ). Allocate memory to instance is 26214MB. Can some one help me on (HowTos)