How to increase frame-rate in the game (for iOS)

I am new in unity. I am creating a game in unity for iOS.

In my game I have a character who fly like a bird on hilly area (terrain) and player need to prevent its collision from the obstacles coming from front (from Z axis) . My problem was that my game contain a number of objects (50-70 ) ,so the frame rate was decreasing. I had just placed the each object in the scene manually. But after that frame-rate was 5-7 (on 3GS iPhone ). So

1). I made some changes and created the new objects dynamically (which were not visible by user at starting ) via coding (created closer objects first and then others after destroying the previous objects ) . Then my frame-rate got increased 5-7 To 7- 12. The frame rate is still too small . My scene have 20-30 objects at one time .

2). I also tried to make the the hills and mountains on terrain of lower qualities ( by increasing pixel error from 5 to 40 and Base Map Dist to 0. beyond it terrain doesn't look good ) and got frame rate10-15. but still I couldn't get the desired frame rate (22 - 30). Then I thought to divide my Terrain into smaller parts and then make them visible dynamically ( as i hav done with my objects ). But The issue is that our character is flying at high from where user can see the whole terrain So I dropped that idea.

 Is There any solution guys. Can fog-effect in unity provide me some aid . Is there any other factor on which frame-rate depends except resolution of objects & textures and number of objects per scene. Some of my objects have the animations. 

Please guys Help me out