Ok, I recently bought SS and well... IT WAS AWESOME! So yeah, going through the game I picked up a few tips that WILL be helpful to the people who have just started.

Once you reach Goldenrod city, go north and you will be at national park. Go there on a Tuesday, Thursday, or Saturday. The bug catching contest will be held on those days and is a good chance to get exclusive (The most exclusive ones are Pinser and Scyther) pokemon that you won't be able to get anywhere else.

Early in the game, be sure to get an ice type pokemon ASAP. You will evencually need it for Champion Lance (Lance has all dragon types and ice is super effective against them).

Once you aquire the Pokeradio, listen to Mary and Prof. Oak's Show to hear about exclusive pokemon that day and their locations.

When you meet Raikou, Entei, and Sucine (Spell check) in the burned tower, Entei and Raikou will roam around the Johto region and the best way to catch them is to use a quick ball or use the move Scary Face to prevent the Pokemon to flee (Sucine will appear in various places in Johto and will move on to Kanto after you beat the Elite Four). Same with Latias or Latios in the Kanto region.