Well, I've always used as a good rule of thumb:
When more or less descending straight in (inbound course to the airport from top-of-descent close to that of the runway course), try to be 35 miles out at 10,000 feet above airport elevation and at a speed of 250 kias.
If you're more or less making a base-leg entry to your arrival runway, the 30 miles out at ten and two-fifty works pretty good.
If you're descent path is going to take you on a downwind...that is to say, you're landing in a direction more or less opposite the course inbound to the airport from top of descent... then, you can do 10,000 feet and 250 kias, 25 miles from the airport.
As someone else stated, it takes about one mile to slow ten knots...so, for example, figure on ten miles to slow from 300 to 200 (in level flight, thrust idle). This rule of thumb must be modified with strong winds...headwind or tailwind....but, with a few years practice, you'll impress even Boeing test pilots..
(joke...)
Using slats/flaps as a speed brake is contrary to Boeing...beats the hell out of the slats/flaps...and adds little drag until you have a bunch of flaps out (i.e. Flaps 25 or more). Boeing has charts and graphs that will illustrate this... Best to use the speed brake first, then gear...or if you really need drag, use both speed brake and gear....and work out your slats and flaps on a normal speed schedule. While some swear by Flaps 5 at 250 kts...there is no aerodynamic basis for this technique. Boeing is correct...After over 12,000 hours in the left seat of those Guppies, I'm convinced Boeing is correct.
Yes, the 737 requires a little more descent planning than many other aircraft, but it's a joy to hand fly...an honest machine....takes a licking and keeps on ticking.
Have fun flying one of the most popular airliners in the world.
PantLoad