Plank - Once you have your body in a long straight line, without moving your hands, rotate your hands outwards and into the floor as if you were tightening two jars. This will automatically tighten your upper arms, .pull your elbows in slightly, and squeeze your shoulder blades together. Then tuck in your ribcage, press your shoulders away from your ears and press your heels back, Now you've got almost every muscle activated, and your standard plank is a full-body/high-tension plank. (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 dropping the head makes the exercise easier. A hanging head also reinforces the dreaded "forward head poke" position (mostly caused by peering at our cellphones). For push-ups, and almost every other exercise, keep your ears continually aligned 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 today 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, pull your navel in and lift your ribcage to brace your 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.