The most important principle for using trad protection, especiallly but not exclusively if you are just learning, is redundancy. The idea is to develop a system you trust while maintaining a healthy scepticism about the reliability of any one piece. Try not to put yourself in the position of having a single piece, no matter how "bombproof," between you and disaster.
Placing more than gear than seems to be essential requires discipline and endurance, marks of a good trad climber. Failing safely is a better longevity option than betting the farm on a single piece.
Nonetheless, all climbing to some extent, but trad climbing intrinsically, involves risk. A lot of climbs have places you better not fall from, and this is part of the essence of trad climbing---performing in a cool and controlled manner when confronting a risky situation. Neutralizing danger, not just by protection skills, but also by climbing skills, is part of the game. (Unadulterated difficulty unencumbered by concerns of mortality is the province of sport climbing.)
Arguments about whether or not falling is a good idea always have these provisions: "if the gear is bomber, go for it," which is fair enough, but such pronouncements avoid the real problem by defining it out of existence. Many accidents happen when the bomber gear turns out not to be bomber. The climber (1) misjudged the pro (something that is quite possible for experts, let alone beginners), (2) failed to build sufficient redundancy into the system, and then (3) misjudged their ability and went for it in a situation when they were not well protected.
As for judging pro, I concur with the posters who recommend aid climbing. Redundancy is a state of mind combined with the will to carry it out. The most difficult issue is how to climb without falling when falling is a bad idea. (For example, if there is one piece between you and the ground and you can't back it up, then falling is a bad idea.) Here I think modern trends can inculcate bad habits. Gym climbing, sport climbing, and bouldering all emphasize moving up in the most marginal of situations. There is a risk of developing a tunnel-vision mentality that, first of all, accepts marginal moves even though the consequences of failure are catastrophic, perhaps not even noticing that the climber has gone from control to high risk status, and secondly, that blinds the climber to both the need and the opportunity to climb down to rest, regroup, and yes, in some cases, to retreat. Mental discipline is the primary tool for avoiding these situations, but this discipline is not something acquired in the gym or on sport climbs.
Here are some exercises that may be of some use:
(1) When climbing in the gym or on sport routes, try to be conscious of how marginal you are. (This does not mean reducing the difficulty level, just striving for heightened awareness.) From a trad perspective, falling may be ok, but an unexpected fall is not good. Know when you are on the edge.
(2) A lot of falls on steep ground happen when the leader runs out of gas. Try to develop a sense of your "half-way point," because this is one of the moments when you have to decide whether to move up or down. For example, a gym exercise is to select a challenging route and then see how high on it you can get and still climb all the way back down without falling.
(3) Develop the mental habit of filing away "retreat data." This can make the difference between stepping down and falling. (For example, when you step over a small roof, the holds underneath disappear. Did you make a mental note of features above the roof that will help you locate the holds underneath?)
(4) Don't neglect the building of a base of climbing below your limit, climbing in which you are relatively comfortable but are also frequently in the "must not fall" zone. A steady diet of well-protected hard climbing at or near your limit, while essential for raising your climbing level, may shortchange you on control and calmness when things get dicey, as they will, sooner or later...