clay bar will take out contaminates in your paint, there are different abrasives in clay bars as well. i have used meguiars, mothers, and clay magic.. i like clay magic myself.
youll see bits of grime, and stuff pull off the paint and to the clay that you didnt even know where there. it will make the paint feel alot smoother just after using the clay.
after the clay is used, all the little minor specs in the paint where those particles you removed used to be, leave pits (you wont be able to see with your eye) well in order to achieve maximum shine you need to level the paint out. depending on how bad your swirl marks are as well, which are yet another mar in the paint, will determine what you use to level the surface with.
you can start out with some minor abrasive polishes such as, poor boys ssr3.. if that doesnt remove the swirl try something a bit more harsh like meguiars #83, or menzerna IP... you may need to use something more agressive yet like menzerna powergloss or meguiars #105...
those you would apply with a porter cable and a foam polishing pad. there are several pad types as well.. that range from a real abrasive cutting pad, to a finishing pad... use what suit your needs.
now if you really need a make over.. you might need to use some compund... i prefer 3m perfectit 3
use with a rotary buffer and wool pad for maximum cut. careful not to burn through the paint.
lets say you have to start with the compund and wool pad.. your not done after that. you would then have to work your way through the polishes as well. each compound and polish will leave behind small mars in the paint.. and with each step they will become more invisible. and the clarity will show through.
its kind of like trying to polish a piece of metal. you can sand with a corse grit paper and then apply the polishing compound, will it shine? yes but will it be clear no.. so you have to keep using finer and finer grit paper to minimize the scracthes the paper leaves behind.
its not that dramatic but you get the idea.
ok anyways after your polishing is done. you want to make sure you protect that surface.
i find a great combo is a nice 100% carnuba 2 coats, followed by a synthetic wax 1 coat, and 2 coats of optiseal work wonders, gives great protection and leaves a nice deep gloss...
and using a ton of wax/sealants isnt always the best, only so much can be absorbed into the paint.
invest in a couple grit guards, and use a few buckets when you wash your car.
i keep one bucket for jsut the wheels
another for just the top half of the car
anotehr for the bottom half
and a few buckets of just water to clean the sponge
same goes with the sponges
have a few of them and dedicate each one for a certain task.
speaking of sponges, get a nice chamile sponge or napped mocrofiber sponge something with a thick nap to it, that will trap dirt and not harm the surface of the paint.