hey, i would add some colour variation (within the colour fields), and maybe hand paint over the black lines to make them look less freshly painted. the white really pops out and looks new, so you might want to paint dirt on those straps.

if you can't find the brush you like, then make one and just experiment with the brush mixer. also, you can download premade photoshop brushes (google "nagel series"). might help.

but it all depends on your style, if you want to go cartoony or realistic.

good luck!