Have you considered incorporating strength and cardio intervals into your routine? Some people tend to see results quicker by varying the intensity throughout their workout. For example, for a one-hour workout, you could try something like this:
5:00 warmup — walk on the treadmill or go easy on the elliptical.
3 sets of 10 bicep curls w/barbell or dumbbells — choose a weight heavy enough that you work your muscles to failure at the end of each set; take a quick break in between sets but no more than one minute.
8:00 intense cardio — you could run on the treadmill, or try the elliptical or cross trainer. Focus on really getting your heart rate up and breaking a sweat.
3 sets of 10 tricep dips off the bench (have somebody at the gym show you how to do these or do a Google search). Again, no more than a 1:00 break between sets.
8:00 intense cardio — try something different from your previous cardio interval.
3 sets of 10 squats w/barbell or dumbbells — again, pick a challenging weight and have somebody at the gym help you with the form so you don’t injure your knees.
8:00 intense cardio
3 sets of 10 lunges — if you’ve never done these before, try them without weights until you’re comfortable with the form. Again, have somebody at the gym help you with the form.
5:00 moderate cardio, followed by 3:00 cooldown
Finish off with some ab work (lots of ideas online) and stretching.
You could do this once or twice a week and vary the strength segments — try shoulder presses, lateral raises, push-ups, overhead tricep extensions, and also try using the weight machines in addition to the free weights.
It could be really helpful to hire a personal trainer for just one session to coach you on form and give you ideas for different exercises. The main idea, though, is that you need to keep changing up your routine to keep your muscles and your mind from getting bored! Don’t get stuck in a rut or you’ll just hate going to the gym.