Originally Posted by APK
Your body adapts to routines after a while, regardless of how taxing those routines initially were. You shouldn't always be striving for that burn, since soreness doesn't automatically equal progress. Regardless, you should be making some significant changes in your routine every four to six weeks to keep your body guessing. You should also try to slightly alter your routine every trip to the gym so that your body isn't able to adapt so easily.
+ a trillion.
Take my routine for example: I work out bis and chest twice a week. One day I'll do upper chest, another day I'll do lower chest, and the next time I go I work on my center chest. Cheap time I use different machines / exercises that constantly keep my body guessing what will come next.
If you have the ability to gain weight rapidly (not fat weight), then the above mentioned routine should get you relatively bigger in a shorter span of time than you would otherwise seen in a more 'stable' routine.
If you are like me however and have trouble gaining weight, it's going to be a much harder uphill battle.