Same here - Took me about half a year of line flying to figure out how a good descent works - now, 4 years later, I do not really plan the descent any more or think about it, and it works out fine (well, most of the times...)
At some point in time your brain will subconsciously integrate all the information you need like DME, green banana, wind indication and so on, and combine it with the "stored" data on how the aircraft performs and just tell you "Now it's time to descend" or "Better use the speedbrakes now"
What I did in my training and worked for me, was to calculate my descend with 290 knots (that is around 3NM per 1000ft in LVL-Change), so I have a reasonable margin to adjust speed between 250 and 330 if I'm too low or too high. Add 3 miles for reducing from 290 to 250, another 3 miles from 250 to 210.
Another trick is to use the banana:
If you are on a straight in approach just put the banana 3NM ahead of your glideslope intercept when you are at 210kts, 6NM when at 250kts and 8NM at 280kts (assuming no wind conditions). That will give you a reasonable margin to slow down. If you are on a downwind you have to anticipate at what point you are 3/5/8NM from intercept and just put the banana there. When you get more experience you can reduce those distances, but that might require some cheating like going above the glide to configure if you made a mistake.
And always remember the 3 problems of a 737 pilot:
1. Too high
2. Too fast
3. Too close
Edit:
Almost forgot: If you are hot&high on the glide FLAPS 10 is a wonderful flap setting - with 200knots the best speedbrake you can get on the 737 - but do not use it that way when instructors are watching you
