One of the fastest ways to build more muscle is to start focusing more on the weakest muscle groups of your body.  Since weaker muscle groups are likely underdeveloped, there is plenty of room for improvement!

If you think that some of your muscles may be lagging behind, here's where to start.  First, try to determine the most neglected muscle groups in your body (the leg muscles are often under developed).  These are areas that have been neglected in your weight training routine, and you may already have a good idea as to which muscles they are.  

If you don't already have an idea which muscle groups may be lagging, give this a try... 

1.  Write a list of each muscle group, ordering them from weakest (least developed) to strongest (most developed). 

2.  Make a second list based on your current training method.  List the muscle groups under each of your training days, in the order that you train them in.  For example, if you use a three-day cycle, you will use three columns.

3.  Compare the two lists. 

Does the order of the lists appear similar or different?  If you find that your current weight training program reflects the prioritization of your weaknesses, then the muscle groups that appeared high on the list should appear either early in the training cycle (on day one), early in each training session, or both.

Now it's time to give these muscle groups a little extra attention.  Start focusing on them more while you train them.  Put a little extra effort in during each set.  You may want to increase the volume at which you train them slightly, but this might not be necessar.  Simply training that muscle group in the first set of your first weekly workout may be enough to ignite the spark they need to accelerate growth.