I'm guessing by hand? Autoglym SRP, super resin polish is very good, bit of snobbery thrown around, but, don't listen; it's great stuff. It wont cure swirls as such, but has fillers that will reduce the appearance of them.
Get on over to DW, we don't bite, but you will get hooked!
After you have cut it do you just use each bit once?
Depends, I know, not much of an answer. Stick it in a bucket of hottish water to get it pliable, get your hands in and squidge it about a bit,spread it out, I usually work with it about 75x50mm at whatever thickness that comes out, when the face you are using gets manky, turn it over, when that gets manky, back in the bucket, fold it and manipulate it untill you get a clean face to work with, start again. If you keep doing this to the point that you cannot expose clean material, it's time for another bit.
If a square is still quite clean after a whole car I stick it in clingfilm and use it later for the inside of wheels etc.
So, yeah, one bit may not be enough if your car's very contaminated, depends. If you have a lot of tar it will eventually remove it, but better to use Tardis and get it off before you start, otherwise you will clog up the clay a lot quicker.
Remember! If you drop it, bin it! Unless you're claying in an operating theatre, you will pick up dirt and grit, you don't want to grind that into your clearcoat.
Has anyone tried spraying the clay bar with tar remover? I did it once by accident and it actually removed the dirty layer to leave a clean contact area.but i had to put the bar in cold water to make the contact area hard again and then back into the warm water to make it soft. It actually worked so i had a clean clay bar for the alloys. Might help when trying to make one last longer. Or it may well be someone will tell me it is a bad thing and not to do it again