What is the right technique for a strength training repetition?

The positive phase of contracting the muscle should last 2 seconds. Pause at the end of the contraction for less than 1 second and then lower the weight into the negative phase of the repetition (when the muscle elongates) for 3 seconds. A full repetition should last no less than 5 seconds. Remember to inhale during the positive phase and exhale during the negative phase.

Time the phases by counting out loud. At the very least, you will need to count in your head when you first start to exercise, but I recommend counting out loud until it becomes automatic. Don't cheat yourself by doing a quicker, easier repetition. Take a breath, 2 seconds up while inhaling, pause, and 3 seconds down while exhaling. Repeat. Work your muscles, and your joints will thank you.

The right technique will vary on the exercise being performed.  If you are completing a lower body exercise and are for example completing a squat then you want to be sure to bend at your hips and knees and try to focus on sitting back as opposed to leaning forward.  You need to also keep a flat back so you are not rounding forward.  In order to complete exercises correctly, you must avoid locking out joints such as your knees and elbows.  You can modify any exercise by adding or subtracting resistance.  Exercise can at times be a challenge to perform certain movements but each movement has an exercise designed to be easier or more challenging.  Be sure to focus on proper form first and then focus on completing the repetitions second.