+1... Practice

When I'm coming off a week or more of non-bike-commuting I find myself back-tracking from the garage to the house or catching myself several times before I leave the house, for forgotten things, before I'm really ready to leave. When I'm commuting 4-5 days a week the departure routine becomes second nature and goes really smoothly.
The other thing is, just accept there is a minimum prep time, and don't try to cut it shorter than it realistically can be. Make it up by riding faster...