i charge my clients for updates once the project has gone live (once you go live, proposal was complete). i charge per hour and bill work in 15 minute increments. anything that takes longer than X (for most clients that's 2-4 hours), i give them a quote. anything less than X, i just do and then send an invoice every 2 to 4 weeks.
raise your rates, you're not charging enough. two simple ways to tell if you are not charging enough:
1. everybody is willing to pay your rates - nobody ever says, "no"
2. you are not making any money