If you'd asked me this back in the 1970's I'd tell you that I took two hits of blotter and just let my mind go. . . but as Dr. T. Leary points out, you only need to do such things once and from then on, simply think and it all comes back. That said, I have to say that it's my twisted imagination that let's me develop material. . . Just yesterday, I was tinkering with an old Artist Kit I've had for years, thought the case was neat so I snagged it out of some curb side recycling. . . I've done nothing with it until now and in playing with the little easel it dawned on me that I could haunt the kit and get a complete Seance out of it. . . an easy half hour show if not more.
This happens to me more than anything but then there are times that I hear a piece of music that gets me on a roll or I see some kids playing with a particular toy . . . ever take a look at those low sitting cabinets made for the new thin screen TVs? They make the perfect mirror base and so was born my girl from TV illusion. . . admittedly, there's more to it than that mirrored bottom but it's still mind-blowing, seeing someone step out of a 2" thin TV Screen. . . but then I'm a bit prejudiced when it comes to using Mirror Box systems, they defy the eye and the mind when things are set-up properly.
Most of the stuff I've had to come up with was product or client oriented, which meant coming up with loads of stuff with cars at one point in time, including Driving a Car through a Solid Block Wall. . . I blame that one on a Cartoon.
In short, inspiration can come from most anything at anytime of the day. I've found that when I try too hard to come up with something, it alludes me. Getting quiet and just allowing your mind to take a trip, really is the solution 90% of the time.