Wearing freshly laundered clothing is forbidden during the nine days. The above applies to any clothing, even if it is not worn on the body, such as towels, tablecloth, or linens for a bed. [Nevertheless those clothing that become overly dirty and sweaty, such as all undergarments, may be worn even if they are freshly laundered from before the nine days. Likewise those accustomed to change their shirts constantly due to being overly sensitive to sweat may do so even during the nine days, even if the shirt is freshly laundered from beforehand.]
Ironed clothing: Clothing that have been ironed, pressed, or dry cleaned in a way that it is recognizable, even after wearing once, one may not wear them as long as the ironing effects remain.
Practically what is one to do with his laundered clothing before the nine days?
One is to wear his freshly laundered clothing [shirts; pants] for some time prior to Rosh Chodesh. Doing so allows one to now wear the clothing during the nine days as its “freshly laundered” status has been removed. It is disputed as to how much time one is to wear the clothing for. One may wear more than one clothing at a time for this purpose. There is however no need to wear underclothing before Rosh Chodesh, as stated above that it may be worn even if freshly laundered from beforehand.
If one forgot to do so: One may place his clothing on the ground for some time hence removing their freshly laundered status. Alternatively one can wait until Shabbos and wear the clothing for some time on Shabbos. However this only applies if it is apparent that he is wearing these clothes as Shabbos clothing and is not merely preparing for the weekday.
 Salmas Chaim 4/4; Kinyan Torah 1/109; Piskeiy Teshuvos 551/17
 Kinyan Torah 1/109; Piskeiy Teshuvos 551/17
 M”A 551/20; M”B 551/44; Levushei Serud
 Ben Ish Chaiy Dvarim 6; Kaf Hachaim 551/91; Yoreh Deah 389/1; Piskeiy Teshuvos 551/17
 Some Poskim rule one is to wear it for a few hours. [Rav Poalim 4/29] Others rule one hour. [Kaf Hachaim ibid; Piskeiy Teshuvos ibid] Others rule it suffices to be worn even for a moment. [Shach Yoreh Deah 389/4 “Some amount of time and not specifically an hour and so is the custom”]
 Daas Kedoshim 389
 Piskeiy Teshuvos 551/17
 Ben Ish Chaiy ibid; Kaf Hachaim ibid; See Piskeiy Teshuvos ibid footnote 115