I did it a couple of times with the rebar, a real thin piece, and I hammered the end flat and I would just hammer and twist it through. When its thin you can kinda "steer" it through. You could get one of those long Greenlee fish drills and use it it, costs a little more than rebar.

 

I went to Home Depot and got the water thingy for about $3 and 20 foot of conduit to do my double wide driveway...
and went under to withn 2 inches of where it needed to come out since I had a sidewalk in front of the house !!!

Very easy and very messy, but it works like a champ !
just be patient and let the water do the work... just pushing and pulling about a couple of inches at a time !

 

i use to do alot of electrical work for a couple contractors and heres how we did it:

dig down on each side of the side walk. use a piece of ridgid conduit and beat it under the sidewalk. be sure the end that you beat on has a cap on it. then there are a couple of options either one, be sure to use UL listed wire approved for underground use, you can come off the ends of the ridgid with plastic conduit or just run the wire under ground and cover it with dirt. but be sure to leave the rigid under the sidewalk incase soemthing were to happen, then the wire will not get damaged.

 

If you go to an Electric supply house you can by a tip that screws onto Rigid conduit for punching thru the soil. Works really great as it pushes and compacts the soil into a slightly bigger hole than the pipe. I understand they have something similiar for PVC conduit, that includes a striking head to hammer the PVC thru. PS make sure you call Miss Utility before starting your work.