LIFT HEAVY...relative to your training experience and physical capacity.
I'm a big believer in the big compound Movements (Squat, Bench, Deadlifts, Overhead Press) and lifting "heavy" has changed my life for the better. However, "heavy" does not have a specific number, it's relative to the individual and the lifestyle they lead. Don't let the numbers I lift or the numbers you see performed by much more efficient and elite lifters intimidate you from these life changing Exercises.
Just a year ago I was doing sets of 5 with 305lbs in the squat, but I was also MUCH more sedentary. Now that I live in New York City, my legs are much more taxed from the tremendous amount of walking I do daily. Of course I'd like to have continued making the progress I was making, but those aren't my circumstances anymore! I use crying about it, you just got to adapt!