As Lou Schuler said in his new rules book...
"Everything works...
Nothing works forever"
Your body will adapt to what ever you do if you do it long enough.
Change your set/rep system, and the type of workout you do (i.e. push/pull/legs, fullbody, chest/triceps, back/biceps, etc...) Every 6-10 weeks.
You'll see the biggest gains in the 1st couple of weeks of a new program. also change the exercises and the order you work your muscles.