Do Not Sacrifice Form for Weight.  It is more important to fail with proper form than it is to just complete the repetition.  I see it all the time, people hoisting the weight up or incorporating different muscle groups to complete a repetition.  When you begin to fail that is the most crucial time to stay true to your form, this is when you tap into the red zone of training and you begin to breakdown every fiber in that muscle group.  It is best of you have a training partner who can assist you when you reach failure. Also when I say failure I mean when you can no longer move the weight at least half of the repetition, without breaking form, then have your training partner give you two assisted repetitions. I recommend 1 set of this type of training per exercise, this will count as a true working set and help bring you gains you are looking for.