Last time I did this, I put the inserted elements into new layers and put some filters to generate some noise on these new layers (w/o applying it to the original picture) until it looked somewhat convincing. You might want to experiment with several methods to generate noise and their parameters (or even combine several of them). Also it can be handy to apply these effects to new effect layers on top of the layer(s) you want to adjust and then play around how you mix them.