All of this comes with the caveat that, as a genderless sort of person, I don't really have a firm grasp on what masculine or manly is. However, these are the things that make me feel most like a guy, whatever that means.
I love wearing ties. I felt quite dapper when I matched my tie to my "date's" dress last Valentine's Day (we went as friends). Unfortunately, I don't own any ties of my own yet. This must be remedied when I have the cash.
I also have a pair of brown leather sneakers that I love. They're the most masculine of all of my shoes. All the rest are pointedly gender-neutral.
Not sure if this counts as clothing, but I have a messenger bag from Chrome that accentuates the flatness of my chest when I bind. The way it goes right across the boob area and helps smoosh it down is lovely, and it's pretty manly as far as bags go.
And my cycling gloves. They make me feel strong and badass. Despite the fact that I ride a cute 70's women's road bike and that the gloves are women's gloves with embroidery I colored to match the bike. On my bike is one of the rare times that I feel strong, and like an athlete.