I think this is an overlooked, yet critical part of your advertising. Think about the type of customers you are looking to attract, young, old, male female, mums, singles etc. Whoever your target market is, your advert, images, description and key message should be aimed at this. Also, if you work in a specialised business, you should still register with some online directories, particularly the free ones as your (specialized) keywords will help your free “landing Page” appear in the results of Major Search Engines.
If you are extremely specialised, you will also need to look into specialist websites that are involved in, or involved in promoting your line of work.