low weight and high reps will only work to an extent.. moderate weight and lower reps get you farther. high sets and reps will train the slow twitch fibers more i believe(if i'm wrong someone correct me plz) and the will firm things up but only to a point . you need to challenge your body to make gains. other wise things plateau. thats just the way your body works, metabloism etc, all factor in. someone jump in here and explain this better than me. i get it i just can't explain it very well i think.. i'm not an expert(far from) this is just what i understand.. with higher weights and lower reps i have seen good progress, i did the whole high reps low weight thing for a long time and i always stayed at the same place, since training for strength i have made more improvments than before in a resonable time frame..