Never try "winging it" when making pitching your ideas. Take the time to understand your concept fully: the characters, the setting, the atmosphere, everything you can think of. The more you've thought of beforehand, the less you'll have to make-up-as-you-go during the pitch.
Additionally, it is entirely OK to have a small sheet of notes when pitching, no one will judge you.
Know Your Audience
It is just as important to know who you're pitching to as it is to know what you are pitching to them. Do some research. If the particular producer or production company primarily makes and is known for comedy movies, you may have difficulty in selling your romantic horror.
Your pitch should be short and sweet (reasonably) depending on your idea. Try to shoot for under 15 minutes so you'll have time afterward to go over any financial information and receive questions.
What are some difficulties you've had with pitching your idea's so far?
So far, I've found pitching to be incredibly intimidating. However, making sure I'm prepared, knowing who my audience is and keeping the pitch short has always helped me in my presentation.