It has been a while since we worked on 3d games, but all that we can explain is listed below.Keep polygons big
1) When you try working with human models, keep the number of triangles as much less as possible.
2) Mobile device doesnt recognize very small polygons if the difference between 2 points is less than a pixel (in decimal) So you model should be relatively huge and boxy, you cannot create a fine shiny model with fine curves when it comes to mobile platform. If you see the 3d game
in this link, the model will have boxes with very bad human anatomy, and this is how it is suppose to be for mobile devices.
3) Keep less textures as possible.
4) Keep terrains in small files and probably try using same terrain again and again to keep loading faster.