The flowbee is a nice product for this situation.
However, not all "made for tv" items work as expected. I have found that using my fingers + the guard that comes with the clippers gives a nice inch and a half length to the top of my head.
There are numerous guard lengths available for many hair clipper brands, usually numbered 1-4 (4 being the longest). Try a "number 4" guard and use a "scissors" pinching motion on your hair, running the clippers with the guard attached across your fingers. The combination of the guard and the fingers may be enough to give you the length you want.
you can also try to mimic the hair salon method of using a comb, but this may take a steady hand to hold the hair up away from your scalp, and then trim along the comb. I like the finger method myself because I can safely rest my hand on my head and know it isn't moving around.