The 8 Limbs of Yoga - Ahimsa
ahimsa pratisthayam tatsannidhau vairatyagah
The more friendly one is, the more one stimulates friendly feelings among all in one's presence
YS II,35 Translation by TKV Desikachar
The first of the 8 limbs of yoga are the yamas and the first of the yamas is ahimsa - do no harm. Do no physical, verbal or mental harm to yourself or to others.
The idea of doing no harm might sound passive (we might think of those Jain monks sweeping the ground before them with a broom, lest they inadvertently kill a bug with their feet), but ahimsa is a dynamic, active, positive kindness. It is the idea that formed the basis of Gandhi's philosophy of non-violent protest (satyagraha), which influenced Martin Luther King and Nelson Mandela among others, and which demonstrates just how powerful, vigorous and world-changing this simple idea can be.
Ahimsa is a good first principle for your asana practice - practice with compassion and sensitivity for yourself. You don't want to sacrifice the positive feeling gained in a pose by pushing yourself so far into it that you feel pain. There is a balance to be found between effort in a pose (the sensation of having muscles work, stretch and come to life) and the pain felt when you crank yourself into a pose with the determination to get further or deeper, but without the self-love to make it work for you and how and who you are today. You make the effort to come to your mat, you commit to focus on your practice, you do your best to make each asana your best version of it, so that it looks like your asana and not someone else's and so that it feels good (challenging, but positive). That's all. That's perfect.
Ahimsa is a good first principle when approaching your inner critic. Most of us have one, don't we? What does your inner critic say to you? Would you ever dream of being as hard on anyone else as you are on yourself? Ahimsa means accepting yourself exactly as you find yourself - stiff hamstrings, weak biceps, tight hips and all - and not wishing it were otherwise or berating yourself for being afraid of handstands or unable to perform cakrasana. In your practice, see if you can be alert to your inner critic: notice when it sparks up it's commentary, listen to what it says to you, you will find that you can choose to ignore it. You might even be able to laugh at it. Decide to be kind to yourself instead.
Ahimsa is the most basic principle for living well. If you're ill, let yourself be ill; if you need to stay in and eat chocolate, stay in and eat chocolate; if you know a long walk will help you today, get outside and do it; if you need to do some yoga, come to your mat. Listen in and be kind. Give kindness to other people. My teacher told me that you can change someone's life by offering them a kind word.
Be kind to yourself and through finding compassion for yourself learn how to be kinder to others, even the people you find difficult; we're all just doing our best in any given circumstance. Even just trying to be kinder to yourself and to others brings more kindness. Don't intimidate yourself by imagining that you need to reach the highest of ideals, we can all think of someone who seems to us the quintessence of eternal sunshine and kindness (the Dalai Lama? Nelson Mandela?), but we're all human, so we all get impatient, cross, grumpy, unreasonable, or frustrated sometimes.
The great thing about yoga is that it lets us be human (with all our mistakes and weaknesses and the dark bits that we'd rather other people didn't know about), but it gives us a place to reflect on how we could have a better, kinder, stronger, more generous way of living and it gives us a method for working on it. It makes sense that it all starts with the concept of ahimsa.