@EpicalAgent56 (View Post)
Use keyboard shortcuts:

• Ctrl+Mouse Drag draws rectangles.
• Ctrl+Alt+Mouse Drag draws squares.
• Shift+Mouse Drag draws straight lines with a thickness of your brush size.
• Shift+Alt+Mouse Drag draws lines and snaps them in steps of 45 degrees.
• Ctrl+Shift+Mouse Click fills the entire closed area with the selected element.

When I say "mouse click" or "mouse drag", I mean left click, right click or middle mouse click, those use different elements.

• Left Click draws with the element highlighted with a red border. DUST by default.
• Right Click draws with the element highlighted with a blue border. Eraser by default, that's why right clicking deletes particles.
• Middle Click draws with the element highlighted with a green border. Sampler by default, that's why middle clicking an element samples it.

Just like how you can change the selected element with a red border to something else by left-clicking the element you want, you can right-click an element to make it highlighted with a blue border to use it to draw when you right click instead of erasing, or middle-click an element to make it highlighted with a green border to use it to draw when you middle click instead of sampling.

I recommend you memorise some of the important shortcuts because those will save you a lot of pain and trouble. There is this wiki page which lists them, and there is this interactive version made by @boxmein.

Oh and since I really like demonstrating things with GIF animations, here is one for you: http://i.imgur.com/MpfpDzM.gif