When doing Bicep Curls for example, pay attention to each of the Three Phases of each Repetition. That is, the Concentric, the Isometric, and the Eccentric. For example, if you take 1 second to fully curl the arm (the concentric), hold in position at the top of the curl and tense the biceps for 2 seconds (the isometric), then take 3 seconds to slowly extend the arm back to the starting position (the eccentric).
It is also important to synchronize your breathing with each rep. That is, Exhale during the concentric phase, and Inhale during the eccentric phase.
(Concentric means contraction of the muscles, eg, the biceps while flexing the arm in Bicep Curl, or the quadriceps in Leg Extension. Isometric means holding with equal pressure to the resistance. Eccentric means relaxing the muscles, eg, the biceps while extending the arm back to starting position in Bicep Curls, or the quadriceps while relaxing the leg back to starting position in Leg Extensions).
A set of 10 to 12 reps done this way with the right weights, will ignite your biceps to flames. 3 to 5 sets of this will give you a much better pump.
Apply this concept to other weightlifting exercises such as Triceps Press downs, Leg Curls and Leg Extentions for better results.