I check the Interplanetary Calculator for the phase angle. Then I load the rocket and timewarp until the solar system looks kinda like it does in the Calculator, and then I switch to something else that's already in orbit around Kerbin and use maneuver nodes to see if that could theoretically go to my chosen destination. If it can, I switch back and launch away. If it's too early, I timewarp some more; and if it's too late, the wall gets a new hole.