in the past i have used oversized channellocks to hold the pin being hammeredso no finger or hands or in the hammer zone

check out OTC tools. They have a hydraulic press thing for this. I'll take some pics at my shop monday for you

I have removed a bunch of boom pins and what usually happens is they get rusted in the static part of the boom they sit in. Lubricant and welding a big nut to them and using about a 1" impact to get them rotate helps a lot. Then there are the ones that have spun and worn a groove in them, they move a short distance and then stop when you try to move them.These need a little finesse to get all the load off of them before they will budge.

As in all repair operations, the boom should always be properly supported when removing boom pins or working on any part of the boom, keys in your pocket and follow manufacturer's recommendations.  

cp1, you could be booming over power lines with the best warrenty an owner could ask for, if the ram/cylinder fails, the first thing asked will be "why are you over hydro lines?"

i've only done pins once and i really hope theres a better way.

cp1, a port-a-power is a ton of help. Weld an eyelet, a fat washer or a link of chain to the pin and apply some pulling pressure. One big smack (without holding back) and your siezed pin will move an inch or more! Dont forget to grease it first and be sure there's no positive or negative pressure on the hyd.ram.