I had this my Nexus 4 for over 3 years and have upgraded to newer smartphone. I wish however to still keep my Nexus 4. I kept it in Diztronic TPU protective case that worn out with time and no longer has that tight fit feel to it. It has accumulated some little sticky residue on its sides and it attracts lint and dust:

I have no idea how to clean it to get rid of sticky residue and everything else that it managed to attract. I tried rubbing alcohol but it was a short term solution and soon I was back where I started. The sides are not made of some smooth material. It's a hard silicone and due to friction it makes it harder to clean. Thanks for ideas.