Well, you could add them up like this:
000 + 001 + 002 + 003..................996 + 997 + 998 + 999
Add the first and last: 000 + 999 = 27
Add second to second last =001 + 998 =27
Add third to the third from last =002 + 997 =27.......and so on for a total of: 1,000/2 =500
500 x 27 + 1[the last digit of 1,000 itself] =13,501 - which is the sum of all digits from 1-1,000.