The next time you are driving and see someone stopped on the side of the road with a flat tire, pull over and offer to help change it. If nothing else you can lend your cell phone or perhaps an extra towel or two to kneel on. Wait with the person until help comes, or help them change it yourself.
You will never have this opportunity to help this person again; this is the only chance you will have to help this specific person on this specific day. Don't pass up this chance! You may find you make a friend for life.
Always carry a car jack, extra towels, water and snacks in your car. That way if you ever run into someone who needs your help on the road, you will be well prepared. Not to mention if you ever breakdown yourself! Make sure you know how to change a tire and teach others as soon as they are able to start driving.
Even if you don't know how to change a tire, you can still stop and offer to help. Perhaps you can call someone to come and get them, or just listen to their plight. No one likes to face difficult situations alone; offer to stay with them until help comes.
Learn the basics on how to change a tire.
Build your own emergency car kit.