The semicolon is one punctuation mark that could really help show your skills and consequently improve your score, especially in the writing module of the IELTS.
Use a semicolon (;)
1) in between two closely related sentences
Example: Ronnie has five cars; three of them are Subarus.
2) before conjunctive adverbs such as: however, also, besides, indeed, otherwise, therefore, in fact, meanwhile, in addition, consequently, nevertheless, next, still, finally, earlier, naturally, and certainly
Example: There were lots of clothes to choose from; however, she was not in the mood for shopping.
3) in between clauses or phrases that contain many commas
Example: Paolo likes a Mercedez Benz, a BMW, and an Audi; Jon likes a Mitsubishi Lancer Evolution, Subaru Impreza, and a Ford Focus; and Alma likes a Toyota Land Cruiser, a Mitsubishi Montero Sport, and a Honda Pilot.
Here's Paige Carrera also teaching us something about semicolons.
Attribution to Rebecca Elliott for her book book Painless Grammar (c) 2006, 1997 and to Paige Carrera
Other punctuation marks:
Parentheses, Brackets, or Dashes