After getting an idea about what your event really needs to be, build up and think of some concepts which would suit the above indications (Time slot, venue, participants, type of event, etc.) Following will be helpful to you as some guidelines.
If your event is set at an old spacious hall, or a chapel, in the morning, daytime, or evening the most suiting decorations include fresh flowers, and other fresh natural resources because the sense of oddness and oldness has to be eradicated. Fresh stuff adds more liveliness to such locations.
If you have chosen a hall, or a room designed with modern concepts, a lot of fresh stuff won’t highlight the decors there’s no contrast between new and fresh stuff. In such venues use more technical items such as crystal-looking fluid lightings, abstract geometric shapes with contrasting fabrics in shades of primary colors, etc. A little touch of fresh flower decorations will be perfect, but as previously said don’t overdo it.
If the venue is outdoors, a temporary hut, or an open space, again fresh items like flowers, leaves, and other natural sources would enhance the authentic look of nature. If the location is on a beach, at evening time use very contrasting colored flowers with colored fluid bars illuminated with lights, and decors made by coconut shells will add enhance the tropical look.
If the location is a hut build in a serene landscape with mist, mountain ranges, and greenery, fresh flower decorations with light shades of colors and ribbon and fabric decors in light colors will be perfect for morning and daytime.
If it’s a night function in a hall, or maybe outside, consider more about light work, dazzling decors, glitter work, and fireworks. An illuminated decoration concept will be perfectly highlighted in the darkness.
An event like a birthday party targeting kids needs more sweet decorations. Morning, daytime of the evening are the best time slots for such events. Remember to include more contrasting ribbon work, crapes balloons, and characters from Disneyland, and cartoons (if they are under age 15). Add some touch of fantasy.
Whatever the decorations which are most suiting the event it’s better to work out them in a specific theme. For example, if the event is in an evening or night near the sea a theme of under ocean would be perfect.
Everything needn’t be pointed out; I think you got a rough idea on how to match stuff with the indications of an event.