Posture
While sitting upright: (5 degrees backward from the vertical). Also, even while holding the handle, ensure that the spinal chord behind the stomach is not concave but convex.
You should be able to keep a slight bend in your elbows and not feel stretched out when holding the handlebars.
Seat height and angle
‘You want to have the bicycle seat set to a height that allows your leg to extend until it is almost completely straight when you are sitting on the seat. There should be only a slight bend to the knee when your foot is on the pedal in the bottom position. This will maximize power and minimize fatigue.’
Seat orientation relative to handlebar
The height and distance of the handle-bar relative to the seat determines posture and weight distribution. A handle-bar too close to the seat post results in a concave bend (rather than convex) in the back-bone, resulting in back-ache.
Saddle should point around 10 to 15 degrees downward - that has been shown to reduce lower back pain by altering the default posture.
Safety
Use helmet and reflective gear. The bike should be equipped with front and rear reflectors.
Cadence
Cadence (pedal-pushes per minute) should be maintained constant irrespective of the rate of rotation of wheels (up or down). This is accomplished by proper use of gears - shifting to low gears when at slow speed (as in climbing an incline) and high gears while at high speed.
Carrying stuff
Carrying heavy items in a back-pack can be strenuous for the back. Carrying them in a front-pack - especially while riding upright leaning 5 degrees backwards is much less strenuous.
Various baskets and racks can be attached to the rear or front axles. Some that attach to only the seat post or the front handle are limited in the weight they can carry. There are also various pouches meant for carrying light items, which are attached to the seat post and handle-bar.
Heavy items carried attached to the handle decrease the manoeuvrability of the bike.
Provisions are usually made to attach racks by providing screw-holes in the frame near the rear wheel axle and under the seat post.
Commute itihAsa
Did a 3-mile commute by mountain bike to work in Mountain view (Jan 2012). Experienced back ache with seat height higher than handle-bar, and at roughly the same level as handle bar. Changed the height to be slightly lower than handle bar, tilted it forward a little - to no effect.
Switched to a hybrid bike for a month. I felt slightly better initially, but eventually, reduced pain returned - necessitating rest after the commute.
Then I switched to sandIp’s road bike (bought at target for 250$) with an aluminium frame and without no shock absorbers. It was so light that I could lift it with my left arm. Greater speed, almost complete lack of pain were the result.
I rode the hybrid bike again after that for 3 days, where I tried to imitate the posture experienced with sandIp’s road bike. Before that, I tried a lagaam to the handlebar, enabling me to ride while maintaining a good back posture (5 degrees backward from the vertical). Also, even while holding the handle, I ensured that the spinal chord behind the stomach was not concave but convex. There was no pain as a result.