Whats the best placement of a lav mic on a t-shirt?
Like this?
http://www.bz1-img.com/images_customers/08/84/34763375_265573_full.jpg
or lower?
Thanks for help!

What does "best" mean?
Least visible?
Highest output?
Least susceptibility to background noise?
Easiest/fastest to rig and then remove?
"Best" for THAT microphone, or for some more high-end mic?

For a female clip the mic between the breasts between the bra and skin..(not between the bra and shirt)
For a male use some medical tape and stick it to the same position as per the female (between the breats at nipple line level)
Where you have the mic placed in the pic it would have changed the tonal quality of the mic.

Depending on the mic you are using and if you don't need to hide it you can turn the mic down onto the shirt instead of up to the mouth. Placing it so close as in the picture can give you too much presence. Brian's advice for hiding is good but not always easy if the person being interviewed isn't prepared for that level of intrusion. Reality shows which hide the mics often have control over wardrobe so other options can come into play.

For quick set-ups, (hidden or not, inside or out) a 'vampire' pin clip is often used on the tee-shit collar. However this can sometimes sound muffled. Otherwise, I concur with the other posts.