How Much Should I Charge?

I need some help in pricing this new "marketing consultant" service I am offering.

Here is the back story - I have been working in internet marketing for about 2 years, building my following and my business in the horse training industry. (I have a video blog and sell online horse training courses.) I recently offered to put together a PLF style launch for another trainer friend of mine who doesn't know much about marketing but does have a large customer base from doing live events. I am just finishing up this launch for her where we sold spots in her online course and monthly membership program. Now she would like me to take over more of her online marketing - managing this membership program (which means editing videos, providing tech support, etc) as well as creating squeeze pages to grow the email list and develop more launches for her DVD set, future courses, etc.

I am guessing it will be 5-10 hours per week, more when we are doing a launch. I will be hiring someone to help with the simple stuff, like editing and uploading video as I am so busy with my own stuff. So after all that... what is the best way to charge her? Bill by the hour? Give her a monthly fee? Hourly for the maintainance stuff and fee based for the launch projects? How much per hour should I be charging? Any help is appreciated! Thanks, Callie