To increase the intensity, have your athletes hold the bottom position for a couple of seconds during each rep.  You'll notice in the picture that almost all of the bodyweight is on the front foot.  It sometimes help to have your athletes put their hands out in front of them to really get the weight forward onto the front leg.  As it get more uncomfortable during a set, most athletes will try to lean backward to allow the back leg to take some of the pressure.   They will also try to stand all the way up with locked-out knees to relieve the tension temporarily.  Coach them to avoid both of these things.

It's certainly OK to do a lunge with the torso upright, but it gives it a totally different feel.  There is a version of a lunge hold where you actually want to keep the body upright with the hips pushed forward to increase the activation of the hip flexors.  That is a different exercise, so please don't compare the two or get them confused.  This is one version that works well for the front leg. 

- Start by taking a large step forward with one foot.
- Rock all of your weight forward so that it is all on the front leg.  Your back foot should be poniting straight forward on the floor, but only your toes are in contact with the ground at this point.
- Lean at your hips to make sure your weight is all the way forward.
- Your front knee should have a slight bend in it and should be directly above your foot at the beginning of the movement.
- Slowly lower yourself down toward the ground until the back knee almost touches.
- Pause briefly at the bottom, then slowly return to the starting position.  Do not completely straighten your leg at the top; it will always have a slight bend in it.
- As you come up, rock your weight forward to make sure the front leg is doing all of the work.  You should almost be able to take your back foot off the ground throughout the movement; it is only there for balance.
- Be sure to perform this movement very slowly and under control.  It should take you 3-5 seconds to go down and 3-5 seconds to come up.
-  Your front knee may travel forward slightly, but it should not go past the front of your foot.  Keep your heel on the ground and push through the entire foot.
-  You can increase the difficultly of the exercise by doing more reps, slowing your speed down, or adding resistance by holding dumbbells in your hands. You can also pre-fatique the legs with other exercises such as squats of leg presses to make it more difficult just to manage the bodyweight lunge.