Keep your eyes peeled on the screen to pick matches. Move the mouse around as much as possible to locate the biggest selection possible. You should also consider changing your browser's zoom settings to make the important section larger. Avoid taking your eyes off the center of the screen to look at the timer.
One of the top tips for Bejeweled Blitz is that the speed at which you play is vital.
When starting, as soon as you see the screen you should begin to make matches, rather than waiting for 'Go'. The quicker and longer you play, the more chance you have of being dealt good hands and getting high scores.