[longbody]

If you were on a bodybuilding program before and gaining mass, then you should change your routine.

Few sets (2-5), few reps (1-5), long rest periods (3-5 min), and cycling heavy weights (80% or better of a 1RM I think, maybe 85%) is the way to get stronger without building mass. Just a matter of training for a neurological effect instead of a physiological one.
Another way to go about about pure strength training is to view it as the practice of lifting heavy weights. You can't lift heavy if you're fatigued, or have too high a volume. Cycling between heavy and lighter weights is required though.

Or you could go for strength-endurance type stuff, like repetition kettlebell lifts, pullups, one-legged squats, without gaining mass.

Diet shouldn't matter at all.