I offer packages because as some have mentioned paying up from means your client has made a commitment and will usually follow through. If they cancel without 24 hours notice it is counted as a session.
I find in just starting out packages work best for me but I do tweak them based on the individual and their situation.
For example I was training a marathon runner to improve her strength and explosive movements as she was training to run up a mountain. She renewed her last package session which took us to about a week before the date of the run. I threw in two sessions for free to help her with stretching and core work. She did awesome at the race and the goodwill was worth it to see her reach her goal. Again depends on your client and their situations. I would not do this for everybody.
Hope this helps,