Particles ahead GUI

Create different camera layers to render things 

Create 3 camera's with the following hierarchy: 

1. Set all camera's to "Clear flags = Depth only"
2. And set the depth of the camera's in the inspector as follow:
    Main Camera: Depth = 0
   Gui Camera: Depth = 1
   Gui Particle Camera: Depth = 2
3. Now you have to create a layer for the GuiParticles:
4. And assign your particle effect to this layer.
5. In you inventory canvas change the render mode to "Screen space - Camera" and drag your GuiCamera to the "Render Camera" field.
6. Change the layer of your inventory canvas (recursively) to "UI"
7. Now finally set the culling masks for each camera as follow:
   Main Camera: Everything BUT "UI" and "Particles"
   GuiCamera: Nothing BUT "UI"
   GuiParticleCamera: Nothing BUT "Particles"
8. You now have 3 render layers with each their own camera. All stuff marked as layer "Particles" will now be rendered in front of the "UI" layer. 

Source: http://answers.unity3d.com/questions/1169898/hi-guys-i-need-some-help-with-gui.html#answer-1169946