Do not and I repeat do not cheat your reps. No jittering, no bouncing, no swinging your arms / legs. If you're not working out your muscles correctly you will get so little.
Look up a few videos online for proper exercise form - anything from bicep curls to squats to crunches. There are small, subtle things about each common exercise that many people neglect which will mean the difference between getting a workout and wasting your time. If you do your reps correctly every time you should not need to lift for more than 45 minutes to an hour.
After that it's about finding out how many sets of each exercise you are comfortable with. Whey protein supplements do in fact improve your ability to perform more sets over time, although you need to work out almost every day to notice a difference. After half a year of taking supplements I find that I'm able to do at least 25-30% more reps than I used to, and that makes all the difference.
Oh, and freeweights are better than guided machines.