I was taught as a child to wear several sets of clothing one after another for a short period of time in the days prior to 1 Av, and I've encountered others who follow the same advice, though the time varies (half-hour, 1 hour, 2 hours).
As I grew older and became responsible for my own time and personal hygiene, I began running into a problem of not having enough time to wear enough clothes for long enough to really feel that I'd de-freshened them without wearing them for an entire day and re-wearing clothes that were now too soiled for my liking. The solution I came up with was to take several sets of clean clothes and put them in the bottom of the laundry hamper for anywhere from an a couple of hours to overnight. This won't soil them, but it will definitely take that freshness out.
If you are a business professional who wears suits every day, a good backup option that works if you find that you haven't got enough sets of "wearable" unfresh clothes, in a year in which the 9 days are broken up by Shabbath somewhat towards the middle (as opposed to being at the beginning and/or end), is to wear different sets of clothing (suits and shirts for men, nice tops and skirts or skirted suits for women) for Friday night, Shabbath morning, Shabbath afternoon, and Se'udah Shelishith, meaning that each set is not worn for more than a couple of hours, preventing it from becoming too soiled, each set is worn for Shabbath (which is key for the legitimacy of this exercise), and each set is now de-freshened for the remaining days of the period.
And might I add - #fwp