As above. Most often it's either the sliding pins that are sticking (due to the grease having dried up or the boot having split letting water in) or the pads not moving sideways in the carrier due to corrosion or gunked up dried grease.
A stuck sliding pin can be a bitch to remove, but a mole grip, hammer and such tools can usually loosen it enough to extract eventually. Usually destroys the boot in the process, though - and is actually a lot quicker if you write the boot off to start with. These are the sort of things I tend to vulture of scrap cars.
As for stuck pads, you have to lever them out gradually, trying not to break the pad material (if they're still of serviceable depth, that is). A chisel is good for this - screwdrivers can give too much pressure on a little area and increase the chance of shattering the pad. Remove the holders and give them a good old clean, get them as smooth and shiny as you can. Then apply a dob of copper grease to the contacting edges of the pads (which you've also cleaned up) before reinstalling.
If the pad on one side of the disc is much more worn than on the other, it indicates a sticky piston. That is, the piston is constantly forcing the pad against the disc on that side. Though not recommended, if the piston boot is OK then you can work the piston in and out with the brake pedal and a G-clamp whilst squirting WD40 or similar under the boot. Doesn't beat removing the piston and cleaning everything up properly, but it's worked for me in the past.
is there an easy troubleshooting method to get the caliper working again? I'm trying to keep the cost down and I don't fancy paying main dealer rates 
and like you say after the car has sat for a while they seize up 
