using a loop makes a more clever algorithm more practical
than within your code at http://www.8052.com/forum/read.phtml?id=152198

compile this with "sdcc -c adc.c"

#define NUM_SAMPLES 6        // 6 == 4+2  (and 10 == 8+2)

__sfr __at(0xab) AD1DAT2;

// combination of glitch removal and averaging
int adc_smoothed()
{
  unsigned int value = 0;
  unsigned char index;
  unsigned char max = 0x00;
  unsigned char min = 0xff;
  unsigned char tmp;

  for (index = 0; index<NUM_SAMPLES; index++)
  {
    tmp = AD1DAT2;
    value += tmp;

    if( tmp > max )
      max = tmp;
    if( tmp < min )
      min = tmp;
  }

  // removing highest and lowest ADC reading from sum
  value -= min;
  value -= max;

  // return average of the rest
  return value/(NUM_SAMPLES-2);
}

and get:

               91 ;------------------------------------------------------------
               92 ;Allocation info for local variables in function 'adc_smoothed'
               93 ;------------------------------------------------------------
               94 ;value                     Allocated to registers r2 r3
               95 ;index                     Allocated to registers r6
               96 ;max                       Allocated to registers r4
               97 ;min                       Allocated to registers r5
               98 ;tmp                       Allocated to registers r7

0000          104 _adc_smoothed:
              113 ;     adc.c:8: unsigned int value = 0;
0000 7A 00    114       mov     r2,#0x00
0002 7B 00    115       mov     r3,#0x00
              116 ;     adc.c:10: unsigned char max = 0x00;
0004 7C 00    117       mov     r4,#0x00
              118 ;     adc.c:11: unsigned char min = 0xff;
0006 7D FF    119       mov     r5,#0xFF

              120 ;     adc.c:14: for (index = 0; index<NUM_SAMPLES; index++)
0008 7E 06    121       mov     r6,#0x06

000A          122 00107$:
              123 ;     adc.c:16: tmp = AD1DAT2;
000A AF AB    124       mov     r7,_AD1DAT2

              125 ;     adc.c:17: value += tmp;
000C 8F 00    126       mov     ar0,r7
000E 79 00    127       mov     r1,#0x00
0010 E8       128       mov     a,r0
0011 2A       129       add     a,r2
0012 FA       130       mov     r2,a
0013 E9       131       mov     a,r1
0014 3B       132       addc    a,r3
0015 FB       133       mov     r3,a

              134 ;     adc.c:19: if( tmp > max )
0016 C3       135       clr     c
0017 EC       136       mov     a,r4
0018 9F       137       subb    a,r7
0019 50 02    138       jnc     00102$
              139 ;     adc.c:20: max = tmp;
001B 8F 04    140       mov     ar4,r7
001D          141 00102$:

              142 ;     adc.c:21: if( tmp < min )
001D C3       143       clr     c
001E EF       144       mov     a,r7
001F 9D       145       subb    a,r5
0020 50 02    146       jnc     00104$
              147 ;     adc.c:22: min = tmp;
0022 8F 05    148       mov     ar5,r7
0024          149 00104$:

0024 DE E4    150       djnz    r6,00107$
              151 ;     adc.c:14: for (index = 0; index<NUM_SAMPLES; index++)

              152 ;     adc.c:26: value -= min;
0026 7E 00    153       mov     r6,#0x00
0028 EA       154       mov     a,r2
0029 C3       155       clr     c
002A 9D       156       subb    a,r5
002B FA       157       mov     r2,a
002C EB       158       mov     a,r3
002D 9E       159       subb    a,r6
002E FB       160       mov     r3,a

              161 ;     adc.c:27: value -= max;
002F 7D 00    162       mov     r5,#0x00
0031 EA       163       mov     a,r2
0032 C3       164       clr     c
0033 9C       165       subb    a,r4
0034 FA       166       mov     r2,a
0035 EB       167       mov     a,r3
0036 9D       168       subb    a,r5

              169 ;     adc.c:30: return value/(NUM_SAMPLES-2);
0037 8A 82    170       mov     dpl,r2
0039 C3       171       clr     c
003A 13       172       rrc     a
003B C5 82    173       xch     a,dpl
003D 13       174       rrc     a
003E C5 82    175       xch     a,dpl
0040 C3       176       clr     c
0041 13       177       rrc     a
0042 C5 82    178       xch     a,dpl
0044 13       179       rrc     a
0045 C5 82    180       xch     a,dpl
0047 F5 83    181       mov     dph,a
0049 22       182       ret

The round trip buffer suggested by Esko is also nice:)

Greetings,
Frieder