icemilkcoffee 

When I started cycling last year I followed a lot of websites' recommendation and leaned forward with a flat back. I got numbness as soon as I went past 20 miles. I found that you cannot just maintain a flat back because you would put too much pressure on your groin. On the other hand you also cannot ride with a hunched back because you would get neck strain. You need to have a 'double curved' back. Here is what you do- get on the bike in the riding position and arch you back slightly. Now use your hand and push in and suck in your stomach as much as you can, while maintaining that slight arch for the upper /chest part of your body. Now your lower back is hunched while your upper back is flat or slightly arched.

Another thing you need to do, is to support your weight with your legs and not your saddle. When you are pedaling hard, this comes naturally because your legs are pressing down hard on the pedals and lifting your butt slightly off the seat with every pedal stomp. But when you are coasting- you need to straighten one leg and stand on that leg. Let your legs be the suspension.