I think I've mentioned FollowUpThen before. It's a perfect, elegant solution to remind you to follow up on certain emails. Basically you bcc one of their email addresses based on the amount of time you want to be reminded to follow up. Well now they've made this easier than ever with their Beta gmail plugin. It basically adds the option to bcc followupthen in every message. I haven't tested it out yet but it looks dead simple. Give it a whirl! Their basic version is free with some premium features for about $25/year.