I don´t know what shader you are using, but you can try to create a copy of it, which will be used for the big parts only and place the following line into pass (probably more than one) of the shaders technique to disable gouraud shading and to enable flat shading for it:
ShadeMode = Flat;

So that it should look somehow like this:
Code:
technique SpecularNormalMapping_20
{
    pass P0
    {
ShadeMode = Flat;
    	 alphablendenable=false;
   	 srcblend=zero;
		 zWriteEnable = true;
       alphaTestEnable = true;
        // compile shaders
        VertexShader = compile vs_2_0 VS_PASS0();
        PixelShader  = compile ps_2_0 PS_PASS0();
    }
   
    pass P1
    {
ShadeMode = Flat;
    	 //blend second pass additively with first 
    	 alphablendenable=true;
   	 srcblend=one;
    	 destblend=one;
		 zWriteEnable = true;
       alphaTestEnable = true;
    	 
       // compile shaders
       VertexShader = compile vs_2_0 VS_PASS1();
       PixelShader  = compile ps_2_0 PS_PASS1();
    } 
    
    pass P2
    {
ShadeMode = Flat;
    	 //blend second pass additively with first and second 
    	 alphablendenable=true;
   	 srcblend=one;
    	 destblend=one;
    	 zWriteEnable = true;
       alphaTestEnable = true;
    	 
    	 
       // compile shaders
       VertexShader = compile vs_2_0 VS_PASS2();
       PixelShader  = compile ps_2_0 PS_PASS2();
    } 
}


(sry, for the ugly formatting, I just copied it from the wiki and pasted that line...)

Lite-C Tut. 1 Lite-C Tut. 2 Lite-C Tut. 3 (en)
Shader Tutorial (ger)

http://www.slindev.com