In general, washing a car will gradually age the paint. This will certainly be the case where you use an automatic car wash or even a 'Polish' hand car wash. This is because they tend to rub dirt into the soft water based paint which causes hairline scratches which build up over time.
Also, these car washes tend to use industrial degreasers to loosen the dirt. That's fine, except that it also strips away whatever wax you had on the car - which in turn means the car will get dirtier quicker and need more washes. Win win for the car wash - but bad news for your paint.
So the best thing to do is hand wash it yourself, using a 2 bucket, 2 noodle mitt method.
Don' t use a pressure hose, it will strip the wax a lot quicker. Wax is your paint's best friend
Use a normal hose to spray the worst dirt off the car.
The two buckets: one for your shampoo and water, one full of clean water to rinse your sponge out into. Soap only one panel at a time, starting at the top and working down. Clean the wash mitt in the clean water bucket after each panel. This gets rid of the dirt so you're not rubbing it back into the panels.
Some people use foam type shampoo with a special lance, I haven't bothered but that doesn't mean they are not good.
The two wash mitts: one for the top panels, one for the dirtier bottom panels. Keep them seperate.
The wash mitts should be noodle-type mitts, they pull a lot more dirt away from the surface minimising rub in. These will do fine: http://www.ebay.co.uk/itm/like/28115...ff14=83&ff19=0
When drying, don't use a plastic blade for anything but the glass. The blade will rub any remaining dirt (including anything that has fallen on to the car or lodged on the blade from previous use) along the paint work, giving nasty long scratches to the clearcoat (lacquer paint, the top most layer).
People also recommend not using chamois, probably for the same reason, but using microfibre towels (soft with a high surface area) to dry the car. This isn't really a problem for me as I don't towel the car completely dry, I spray on Autoglym aquawax whilst the car is clean but wet. Aquawax isn't a main wax coat, but is great for topping up the wax coat, building up layers of protection wash by wash.
Once the car is dry, whenever you have time, apply a layer of a good wax. Autoglym HD wax is fine (a £20 tub will last you several years), there are lots of other good ones out there which people will recommend to you (Poor Boys, Meguiars etc).
As for polishing, this is something you'd only do about every 6/12 months. The wax is the important protective bit, the polish is actually an abrasive compoud that you use to rub the outermost layer of the paint smooth, thus reducing scratches. If you've maintained a good wax coat throughout the year, you shouldn't need to polish often. When you first get a second hand car, that's when you want to polish it, to create a smooth finish on which to apply the wax.
When it's too cold to wash the car yourself, it may be better to leave it dirty than to take it to a car wash who will strip the wax layer and gradually age the paint.
That's not a conclusive list but I hope it helps.