PostProcessing in XNA 4

I am trying to implement a PostProcessing manager in my 3D engine, but I'm failing. I keep getting the first texture just slapped over the top of everything else. I'm just creating a new RenderTarget2D using the GraphicsDevice's Viewport in or开发者_如何学运维der to set the Width, Height etc. but it just overwrites everything.

Does anyone have any ideas? My understanding of the RenderTarget2D was that it took a "snapshot" of the screen and then I can redraw that later.

Ensure that you aren't setting your RenderTarget2D more than once, also use the GraphicsDevice's viewport to create your RenderTarget2D