Two ways, either own/lease machinery or negotiate rates with a lot of plant companies and act as a middle man. The few times I hire now is from the latter, they must have some fantastic rates as always cheaper than direct and even on my account at local hire company and that's to the point where a week's hire was equal to a single days hire for a forced action mixer.
Personally would never hire my kit out.
As Rory said you will attract all the people who will not give a toss about your kit, it will be damaged with quotes along the lines of "it turned up like that" even if the kit was box fresh, off the delivery wagon. Generally hired by the weekend warrior or Bob who is trying to save some money on his extension/landscaping/patio who has never as much as played with a toy excavator let alone tracked a machine. Your kit will be used as a battering ram and just abused and good luck trying to get money back for the damages.
Don't get me wrong there are people making money on it, have seen someone with quite a few bobcats and the large companies all survive so must be some money in it, bit you need all the associated bits, yard/storage (unless you lucky to have your kit go straight from one site to the next), vehicles to transport plus the driver, CPC, tacho (unless you get companies to transport your kit) a fitter, then all the insurance on top
