You may use filtration method according to our previous work:

https://www.researchgate.net/publication/261767953_Highly_Stretchable_and_Sensitive_Strain_Sensor_Based_on_Silver_Nanowire-Elastomer_Nanocomposite

Article Highly Stretchable and Sensitive Strain Sensor Based on Silv...

 I found these ideas to be interesting in some applications:

http://www.ncbi.nlm.nih.gov/pubmed/8662565

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3231992/

Thanks all

If the polymer can be made using a thin-film deposition method (e.g. fluoropolymer by PECVD), than you can co-deposit polymer and metal in a vacuum deposition chamber.  I know people who did that and the distribution of metal nano-particles is quite uniform in the films.

A gas phase plasma sputtering will be reliable, which is the most powerful method If you would like to achieve mono disperse (uniform) distribution of metal nanoparticles on a polymer film substrate.

Specially, you may not only obtain uniform particles but also exfoliate and functionalize them into a desirable medium

Here is one of our work : https://www.researchgate.net/publication/264672738_Smart_Composite_Nanosheets_with_Adaptive_Optical_Properties

Article Smart Composite Nanosheets with Adaptive Optical Properties