Practise is the key to finding good spots for hiding the mic while keeping the clothes noise to a minimum. If you have the chance, try and experiment with gear before you are on the set, as time is limited and placing the microphone can be an awkward situation. Try out different kinds of clothes too.
The best microphones I've used are the DPA 4060 miniature mics. They are a little bigger than the Sanken COS-11 and a lot bigger than the Countryman (which is extremely small), but they sound 10 times better than the others. And they come with a specially designed plastic "concealer" which is really simple to use and eliminates about 50% of the clothes noise. You fasten the plastic thing together with double sided tape to the inside of the outermost layer of clothes.
Whatever the mic, try to get hold of some double sided tape and some medical tape, if you need to place the mic directly onto the skin.
When you place the mic, try to place it in the middle of the chest, right above the heart, where the chest caves in a little bit. If you place it higher, you get more clothes rustle, and if you place it close to the neck you lose a lot of the high frequencies, as the chin shades for the direct sound.
If you are working with a shirt with buttons, or a polo shirt, you can place the mic between the buttons, facing towards the open air. If the actor wears a tie, you can place it in the knot, facing down. DPA has a special foam piece for this purpose, but you Can make your own foam piece if you are using a different mic.
There is a lot more you can do, for instance you can sometimes hide the mic in the actors hair, with a little help from the makeup department, but you'll have to learn by experimenting....