To purchase this Sefer, click here
Lack of concentration possibly invalidates the blessing: 
If one does not understand the words of a blessing or thinks of other matters while reciting it, according to some opinions it is considered as if he did not recite the blessing at all and is hence required to repeat the blessing. This applies even if he said the blessing in Hebrew. Practically one is not to repeat the blessing although initially one must be very careful to concentrate while reciting blessings.
Not to do any Melacha while saying the blessing:
One is not to perform any action that requires concentration while reciting Asher Yatzar. One is not even to learn Torah while reciting it as all blessings initially require concentration according to all. This applies by all blessings.
May the blessing be said while one is drying his hands?
If doing so will not disturb one’s concentration of reciting Asher Yatzar then it may be done. However some Poskim are stringent in this matter even in such a case. Likewise some avoid reciting the blessing until the hands are dry.
Reciting each word with attentiveness:
One is to say every word of the blessing of Asher Yatzar with concentration, holding in mind the great kindness that Hashem does with man, as explained above.
Segula for cure and preventive healing:
The saying of Asher Yatzar with proper concentration is a great Segula for cure of ailments. In the words of the Seder Hayom: “If a person is complete in his attributes and he carefully utters his words he will not become sick all of his days and will never need a doctor. Certainly then one is to say the blessing with strong concentration and a complete mind to the healer of all flesh.”
May one do activities that do not require concentration? It is clearly implied from Admur ibid that doing so is allowed, as the prohibition of “Aray” only applies by Birchas Hamazon and the first paragraph of Shema, and not by other blessings, as rules Admur also in 63/8. So also understands Tehila Ledavid 191/1 in Admur [and hence questions his ruling], and so seems to rule Peri Megadim 191 M”Z 1.
Other Opinions: Some Poskim rule that one may not do any action while reciting a blessing even if it does not require concentration, being that it makes the blessing appear unimportant [Aray]. [Taz 191/1; M”B 191/5; Piskeiy Teshuvos 191/2] See Tehila Ledavid ibid that it is impossible to do a Melacha without concentration and hence questions the ruling of Admur ibid.
 See previous footnote for opinion of Admur and other Poskim in this matter.
 Seder Hayom