How do I set a default email address?

I have four email addresses, three business ones and one personal address. Each time I write a new email it's sent from one of my business addresses and if I want to send the mail from my personal address I have to remember to choose the correct personal account from a drop-down box. Is there any way I can make my personal account the "default" address from which my emails are sent so that I don't constantly have to select from the drop-down box?