In addition to ryanb31 comment, you should only control garbage collector manually in really specified cases (where you set your application in a different state, i.e doing image processing of large file).
You'll need to collect collected items from garbage collector which is another overhead and has to be avoided for normal cases.
Control the GC:
http://msdn.microsoft.com/de-de/library/system.gc.aspx[
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Regards