As a ROUGH guide, I remember these figures (these figures work exactly for a G/S of 360 kts. Going faster? increase the ROD a bit).
1000'/min = 6 x Alt = distance
1500'/min = 4.5 x Alt = distance
2000'/min = 3 x Alt = distance
3000'/min = 2 x Alt = distance
So for the example of descending 10,000 (Alt = 10):
At 1,500'/min you would start the descent at 45NM
At 2000'/min, you would start the descent at 30NM.
You are between these two to split the distance, say 1700'/min. Now that you've started your descent, you can refine it using the FMS.
This is not as accurate as the other methods mentioned but I find it pretty good if I'm given an unexpected clearance to get me descending at an approximate rate.
If I have time to plan, I use kwachon's method.
Hope this helps,
Chips