Compile a to-do list and have an errand day.

This helps by giving you a much fuller look at the locations you need to travel to and allows you to group things more efficiently, rather than driving all over town each time an errand arises. If you decide Thursday is always your errand day, you can tell anyone who needs your time that Thursday is your errand day and plan around it from there. The other benefit is that it gives your brain four days off per week from errands. Have the willpower to only allow errands to drive you one day a week and you get the four other "business days" to yourself for what you want and need to get done.