I started by stuffing socks in my bra. That was fine for a long time. Once when I was in college, I saw a crossdresser at an outdoor Halloween party. It was dark, but the first thing that gave her away was that the socks she stuffed in her bra were coming out everywhere.
Eventually, I wanted to take it to the next level, and I bought forms in a size C for me. I would never go back. I would never dress without my forms, except when trying girl's clothes at a retail store while in boy mode.
The biggest difference: Socks look somewhat realistic; forms feel somewhat realistic. It's a huge difference.
