Forget weight machines, lunges are one of the best ways to work out your core, glutes, quadriceps, hamstrings, adductors (inner thighs), and obliques. Here is how you perform lunges with proper form.
1.) Start with your feet together. Stand tall with your shoulders back and your shoulder blades pulled just slightly together to be sure you aren't slouching. Take a deep breath in and as you exhale pull your belly button in towards your spine.
2.) Step back into a lunge position. Lunge down as far as you are able without any joint pain. You want both legs to be at approximately 90 degree angles, if you can. Your front foot should have the weight evenly distributed between your toes and heal(avoid putting all the weight on your toes). On your back foot, your heel should come off the ground and you should be on your tip toes. Be sure to keep your feet straight ahead and not toed out.
3.) Firmly push through your front leg focusing on pushing through your heal to activate your glutes and come back to a standing position.
4.) As you lunge down and up keep your upper body right over your pelvis and lunge straight down and up. You want to avoid moving forward and backwards.
5.) Be sure to breath. Take a deep breath in allowing your lungs and abdomen to expand, lower down into your lunge. As you exhale and come up out of your lunge, bring your belly button in towards your spine and contract your core.