Whatever arrangement you make, clear communication will make the process more smooth. If your plans change on the day you planned to share a lift or your car, let the other party know as soon as possible. It’s advisable to have a plan B (even if it’s not perfect) if arrangements do fall through for any reason.
If you share your car, don’t forget to agree ways of working such as a way of measuring mileage and who is responsible for cleaning the car after use.
It’s totally ok to ‘break up’ if the arrangement isn’t useful any more. It’s good to be clear about what you both need and if it’s not working out, have an honest conversation about ending the agreement. Many factors in our lives change over time, so your lift- or car-sharing buddy will probably be expecting the arrangement to come to a natural end at some point.