Begin to plan your tour anywhere from 6 months to 1 month in advance of your first show. Earlier is definitely better if you are planning a big tour, but if you are only spending a week or two travelling around your home region, you might be surprised to find lots of opportunities with relatively little effort (if you have been keeping in touch with all your contacts that is).
I always start by deciding where I want to go, both specific cities/venues and general region/route. I have found that tours keep their momentum best when the first and last events are something special (ie Common Ground Fair, May Day Parade, SOAW). Special events can be a nice change of pace somewhere in the middle too. Picking an awe-inspiring route, like following the Mississippi River or travelling from South to North during the spring time, will ensure a memorable adventure.
I use google maps and calendar to help me wrap my head around what kind of route is realistic and/or desirable. I have used both of these tools in different ways over the years, they are very handy and easy to use. Here are a few examples:
Color-coding on the map is really helpful to getting an early idea of whether you are getting a good variety of shows – the ideal tour route includes colleges, community venues, public schools, and special events alternately, to keep it lively and fresh. I always aim for 30% college, 30% community, and 30% public school. Color-coding is also helpful when you need to make a choice between one potential route and another – you can see relatively easily which one has better opportunities. You can use maps to determine if your drive times between shows is reasonable and to avoid planning mistakes that lead to lots of back-tracking.