Plank - Once you have your body in a long and strong line, pull your navel in, press your shoulders firmly away from your ears, and press your heels back and down. Now you've got almost every muscle activated, and it's truly a full-body exercise. (Quick check: Is your head aligned? See Push-ups below.)
Push ups - Before you start and during each repetition, be sure you maintain a straight line from your heels all the way to the top of your head. Many people let their heads hang. This is a common mistake because it makes the exercise easier. A hanging head also reinforces the dreaded "forward head poke" position (today mostly caused by peering at our cellphones). For push ups, and almost every other exercise, keep your ears continually aligned directly over your shoulders.
Hamstring stretch - Whether you're stretching your hamstrings from a seated or standing position, keep your back straight as you hinge forward from your hips. In the Jane Fonda era, the rounded back and nose-reaching-for-the-knee approach was standard . But now we know that hip hinging with a straight back gives a superior stretch and also helps us practise good upper body posture.
Biceps curls - There's more to curls than lifting weights up and down. Before you start, lift your ribcage to brace through the core. Relax your shoulders. Then hold an imaginary piece of paper between each upper arm and your torso. As you curl the weights up and down, don't drop the paper! This approach isolates your biceps muscles and prevents additional muscles and momentum from helping out.