It depends on your age, your diet, and how trained you are. If you already train regularly, you can push harder(a 25% increase in weight or time is really demanding). You don't want to injure yourself. Most trainers would recommend about a 10% increase in weight or time per week. For example, if you can do a set of 100lb squats this week, try 110 lbs next week. If you can run 5Km this or 30 mins this week, try adding .5km or 3 mins. If you use the 10 % rule, in 6 weeks you will be 60% stronger and faster. Like Lutzie said, slow reps really helps. And like Alex said, vegetables- you need them to help metabolise protein, and process the wastes your body produces while exercising. Finally, work hard but rest, too. If you don't sleep, or take rest days(work out one day, and REST the next) you'll over train- you wont give your body a chance to recover and build strength. If you are older, you need more recovery time. If you work in the physical trades, you especially need to rest. to follow a challenging training regime. Good luck, and let us know how you are doing!