You could do an excel spreadsheet or a notebook and list all the info that way. Date, place, hours, type of experience, interesting things seen (or learned), etc. I really think recording your experience as you go is a WISE decision; it will make things so much easier!
Also, don't forget to list awards, clubs, sports, other volunteer work, notable hobbies, activities that you have done, etc. These also have to be listed and can go as far back to include high school. Again, it'll be easier to remember the dates, hours, etc. when the time comes to put them into your VMCAS.