Okay, so far this thread is way top-heavy with post-ops and long-term HRT users. How about some input from those of us in early-early transition? I don't GET to "be" a woman (other than the fact that I AM one) all that often - usually only a few hours a week. So...
In-between (when I'm presenting as male), I first of all imagine my body as female as much as possible. This is easiest when I'm just falling asleep or waking up. During the day, I can usually visualize my eyelashes as having mascara on them, which helps. I only use the urinal when I absolutely can't avoid it. And I avoid at all costs seeing myself in the mirror.
But... when I get to...
What lets me feel most feminine is, of course, dressing. Particularly lingerie and pantyhose. But also makeup, wig, and clothing. Putting on heels is a particularly feminine thing.
I am looking forward to getting rid of my beard, so I don't have to wear as much makeup.