Mostly we use the boat, with some help and some specialized tools and techniques...
My very favorite thing in the whole world is a 13 lb. post maul. Followed in order by very sturdy Yamaha outboard lower units, hydraulic jack plates, and stainless steel props. And I grind down my zinc anodes/trim fins so I don't trap ice between prop and cav. plate. And I like really good bfg tires so I can pull the whole mess back up the ramp (or in some cases, not a ramp) when I'm done.
Procedure goes something like this:
Back trailer down ramp 'til skeg is almost touching ice...
Use maul and spud to make a hole big enough to lower the prop into. Just a couple feet across will do as long as there's a little water underneath. Jack the prop down into the water.
Fire motor and "wash" the ice -- this may buckle the ice near the hole, or might just spill a couple hundred gallons of water out on top of the ice... but either way it's a good thing. You'll hear all kinds of angry noises as the prop chews on the ice.
Once the hole gets bigger and the ice starts to get some water weight on top of it... Get to work with the maul 'til you have a hole big enough to drop the boat in. For us this usually means about 20' across.
Drop the boat off the trailer, manhandle it so the bow's pointing out, and get to work driving up on the ice, breaking a little, repeat. I use the weight of the boat as my first option, getting out and pounding with the maul as needed.
In these pics we had broken about 6" of ice to get the boat in. The two-mile ride out to the spot had 2" ice most of the way out... our path re-froze and had 3" ice on the way back! (I wouldn't usually be so bold but we had an "out" to get us to another ramp if our plan A didn't work... and there were a lot of ducks!).
Later this season in another spot we used the same boat to open up a ramp that had as much as 14" of ice on and in front of it. We only had about 150 yds. to go but it wound up being about a 6-hour job. It was also worth it... but too damn much work to take pictures.
p.s. all this nonsense is how my boat got its name... this is what my wife painted on it for me this year: