Your stem should turn on the steerer if one elephant hits your wheel and another hits the bars, or if the bike hits the ground hard in just the right way, but now with any lesser force.  Before you race, grasp the wheel between your legs and wrench hard on the bars.  If the stem turns, it was too loose.  Loosen it, straighten it and tighten it again, harder*. Double-check the headset for play or sticking at the same time.

Similarly, get in front of the bike and really push down hard with your full weight and a punch on the brake hoods. This is the force the bars experience when you hit a hard bump in a race. If the bars drop, loosen them, put them back where they were and tighten them again*. Pound on the side of the seat. If the post twists in the frame, loosen it, straighten it and tighten it some more*. You'll notice that a lot of new bike parts have torque specs printed on them. I've used a torque wrench a few times to achieve those specs, and bolts need to be surprisingly tight to reach the specs for the most part, especially if you are using a hand-size allen key or allen key set.

Do not get in the habit of just tightening everything on the bike every week though. You can often tighten a bolt just a little more if you push hard enough. That doesn't mean the bolt was loose. If you keep tightening a bolt that already seemed tight, you are actually stretching the bolt. Eventually you stretch it past the "elastic limit", meaning that it actually begins to deform permanently. Once that happens it breaks just a hair of tightening later, or maybe when you hit a bump just the right way.

* If you have a torque wrench and parts with specified torques on them, tighten bolts to spec. If the parts are still not secure, toss them. If you have carbon-fiber parts, get a torque wrench that measures in the suggested range.