Center your subject in the frame with a visually distinct background at least 15 feet or more behind him. Slowly move the camera toward or away from your subject while zooming in or out. The background will appear to shrink or grow disproportionately to the character, creating a visual paradox.
Have a crowd of extras move aside one by one at the last possible second while zooming into a central character; this will give the illusion that the camera is simply moving, untouched, through a crowd.
Pan the camera across an empty space while zooming in, landing with both effects on a single character or grouping of characters. This gives the audience a feeling of "sidling" up to the character(s) sneakily. This effect is great for tuning in on private conversations.
Move the camera very quickly across a crowd and zoom in sharply on your character. This gives the double impression of the character being in the midst of a huge throng, while also making her the most important member of said throng.