Strength Workout Guidelines

Before you start

You should be injury free. Never exercise an injured muscle - it needs time to recover. But if you have a minor injury to a muscle that is not being targeted or that does not provide stability for any of the planned exercises, then by all means exercise the healthy ones.
You should be rested and recovered from the last strength workout.
You should be well nourished and ready to work intensely for a relatively short period of time.

Posture, balance and flow

Speed

Learn to perform the exercise slowly, with perfect form, before challenging yourself with more resistance or speed.
Add weight or speed gradually.
Minimize the use of momentum.
Activate, work and fail the maximum amount of muscle tissue by moving slowly.

Breathing

Maintain a deep and steady breathing pattern.
Do not hold your breath.
Do not make a habit of exhaling as you work to lift the weight and inhaling as you lower the weight. (See Engage, below.)

Engage

Keep the target muscles under constant tension when both raising and lowering the weight.
Flex the target muscles through the transitions between raising and lowering movements, and vice versa. In other words, do all that you can to not unload the target muscle during the exercise.
Do not use momentum, as that will create weakness in the muscle tissue that is being bypassed.
Do not release tension until the muscle fails.

Push the limits, feel the burn

Keep the target muscles under tension until you can no longer move the weight - and then try to move it for 10 more seconds. You should feel a burning sensation.
The burn is your muscle tissue signaling the rest of your body that you need get stronger in this area. That signal is necessary to rebuild the target area, stronger.
Learn that the burn is only temporary; that it will go away as soon as you release tension.
Do not confuse the burn with injury pain.

Work to failure in about 90 seconds

More time under tension is wasted, and may stimulate a different response than the one we want ("build endurance" as opposed to "build strength").
Less time under tension probably means that you have not activated, worked and fatigued the target muscles adequately.

Rest

The high-intensity exercise that you perform to build strength does not by itself make you stronger; it only creates a stimulus to strengthen the failed muscle tissue. In fact, the exercise itself makes you temporarily weaker. The strengthening process takes 48 hours or more, depending mostly on age and fitness level.
If you ask too much of the failed muscle before it has had sufficient time to rebuild itself stronger, it won't.
High-intensity exercise of a target area during the recovery period will not be effective, and will halt the rebuilding process - so both workouts will be wasted. And you may be injured.
High-intensity exercise of any part of your body during the recovery period may "detrain" your body's natural recovery and strengthening response.
Low-intensity exercise during the rest period is beneficial.

Plan

Perform strength workouts at least once a week, except during recovery weeks or during a taper (you should perform no high-intensity strength workouts during those periods).
Schedule strength workouts with as much time as possible before the next workout that will make high demands on the target area.
If you perform one strength workout a week, the best time for it may be on the evening before your rest day. You DO have a rest day in your microcycle, right?
If you perform separate upper and lower body strength workouts, you should schedule the upper body workout at least two days prior to the next high-intensity swim; and the lower body workout at least two days prior to then next high-intensity bike or run.
Traditionally, triathletes get the most strength work done during the off season or during a base training period.