I was taught this when I flew the L1011, it worked on the B757 and now the B737. Caution: this is technique only.
Take your altitude loss required, ex. 30,000 - 10,000 = 20,000.
Divide by 1000, ex. 20,000/1,000 = 20.
Multiply by 3 = 60. Now 60 miles is the distance needed to descend 20,000'. In calm winds. Start your descent at least 60 nm prior to your crossing restriction.
Now, I'm sure you're asking, "but what speed?" It worked for me for Mach .84 to Mach .82 to Mach .78 with a transitions from 340, 320, 300 kts. because...
... you should plan to descend at a rate 5 times your ground speed. In calm winds a B737 will by going around 450-500 kts. 5 x 450 = 2250 fpm. This is roughly what a B737 will descend at.
So, it takes 4-5 nm to decelerate from 300 to 250 kts without using speed brakes. It takes about 4nm to decelerate from 250 to 210kts Try to plan NOT using speed brakes.
So (again) add this 5 nm to the 60 planned above, and you should theoretically be at your altitude 5 nm prior to the requirement. If you have to slow, you can slow down after you level off without speed brakes. With the desired altitude set in your MCP, use your altitude trend reference symbol, i.e. the "green banana," on our ND to monitor your progress.
Now, if you're trying to slow down while descending, know that the A/P is not very aggressive. Whether you're in LVL CHG or SPD INV in VNAV, the A/P will aim to descend at 500 fpm, which will allow it to descend and decelerate, theoretically.
Here's another gem. If you're in VNAV or with GS CAP on a 3º glide path/slope, you can maintain 250 kts until 12 nm out, calm winds considered, then you can throw your gear out, slow down and configure on speed to be stable by 1000' agl. The 1st time you try this, use the speed brake with the gear to slow and configure. This should give you confidence.