You can build up your physical strength through the use of lighter weights. Your muscles will have just as much force as when you lift heavier weights, except you will be going much slower. These are especially great for bench-presses. Go with about 40-60% of what you usually lift and do 8 sets of 12 repetitions pushing the weight up quickly. Have a 30 second rest period between sets.
To focus the efforts of your reverse crunches and hanging knee raises to your abs make sure you round your back forward. More specifically, round it by doing a forward roll of your pelvis and hips toward your chest. If you raise your legs instead, you will just be working out your hip flexors.
Commit to your workout by starting early in the day. Some studies show that exercising in the morning will give you an increase in energy throughout the day. If you add a set of exercises to your morning routine, you may be more likely to do them on a regular basis.