I use a big-*** hose clamp and fit jugs with the pistons already on the rods.

 

Ditto Captain Itch ., a large hose clamp will do the job as well as an expensive ring compressor . Be sure to locate the hose clamp adjuster screw at any point where the rings are solid and not at a gap in the rings . Tighten it just enough to let the rings slide in the barrel .
Also... be sure the piston ring grooves are clean , ensure you install and gap the rings on the piston according to specs.
Be careful , the rings are very hard and break easily !

 

what some have said....

pr compressors are useless IMO... i simply do it the way Dan89 and bluharley do it

however, i would very much recommend a proper wrist pin clip tool....george's sells a nice one... it's what i use

 

I use Teflon buttons instead of clips. Never had a failure, and you would laugh at how simple it is to install. Got them from Axtell.

 

I used a $10 ring compressor from OReillys.