Success is going from one failure to the next without losing enthusiasm.
~Churchill~

“Work to failure” is the muscle conditioning mantra. It has long been believed that this is the only way to muscle gain. Other research has suggested that one can still achieve muscle gain without always having to work to failure.

http://www.trainingscience.net/?page_id=635

Training to failure means doing an exercise repeatedly until the muscle being trained can no longer perform the movement. Though this seems to be the standard approach for muscle gain, it can cause undue stress on the system it attempts to make stronger.

Muscle conditioning exercises done with proper form engages all types of muscle fibre. Other systems get trained as well (e.g. neural, hormonal). Rest and recovery is as important to muscle gain as intensity of the exercise.

As a general guideline, one should perform 8-10 exercises for the major muscle groups two times per week. (This is great news for those short on time and busy travellers!) For each exercise, perform three sets. Either rest or do stabilizing exercises between sets. To keep the muscles (and you) from getting bored and hitting a plateau, vary the following:

• choice and order of exercises
• equipment used
• speed, intensity and number of reps
• recovery between sets and sessions

Variety is the spice of life and muscle training. Balance working to failure and not working to failure with rest and recovery and keep changing up your routine as you get stronger.