The strong scents of plants come from aromatic oils which are in plant cells. You can release these oils by soaking the aromatic parts of the plants (they must be dry) in alcohol (for one week), or soaking them in mineral or olive oil (for one-two weeks).

Aromatic (highly scented) flowers, herbs and spices, essential oils and incense have been used by people living on this continent for many centuries. There are 200 commercially produced essential oils that are obtained from oil-bearing leaves, flowers, bark, seeds, or wood of aromatic plants. Natural scents are a complex mix of chemical components. The primary organic compounds are terpenes. They are extracted from odoriferous (strong smelling) things through a distillation process that passes steam over the plants, evaporating the volatile compounds which then condense in water droplets.

To find out more about Fragrances and their chemistry, check out these sites!

Microscapes: the fragrance collection
This site has some dazzling color pictures of the molecular structure of perfumes.
http://micro.magnet.fsu.edu/micro/gallery/fragrances/fragrance.html

Potpourri
This commercial herb site helps walk you through the basic steps of making potpourri.
http://www.frontierherb.com/herbs/potpourri/classifications.html

Natural Pesticides
This commercial seed site describes plant fragrances that can be used as natural pesticides
http://www.seedman.com/pest.html

Chemistry Lab Experiment for making synthetic perfume
http://www.iit.edu/~smile/ch9205.html