Drawing rectangles

If you're drawing on a computer, the program you're using probably has a tool for drawing rectangles. If not, there's usually an easy way to make straight lines and thus making a perfectly straight rectangle is rather easy. However, most of these tools only produce squares and rectangles that are perfectly straight up and down. If you don't want it to be like that, I'd suggest drawing with angles instead.

When it comes to drawing rectangles by hand, the main problem isn't making sure the corners are aligned: it's ensuring the lines that make of the sides of the rectangle are straight. A very helpful trick is to tilt the paper or position yourself so that the sides of the rectangle are completely vertical and horizontal from your perspective. It's easiest to draw rectangles when their sides are going up/down and left/right, so don't try to draw them diagonally if you can avoid it.

Drawing Diagonally
Kinda hard to get right

Drawing Vertically
Much simpler

How to draw a rectangle by hand:
Start out your rectangle by placing a mark where each of the corners are going to go. Note that if you're drawing on a grid, just use the corners of the grid to mark your corners.
Use your pencil or ruler to check that they are where they belong. For example, the bottom left corner should be directly below the top left corner and directly left of the bottom right corner.
Simply connect the marks using straight lines to finish.

Tricks for making a perfect square:
Trick #1:
If you're working on a computer, you may be able to use the rectangle tool to draw a perfect square very easily: just hold SHIFT while you drag the mouse. In the picture to the left, the red line is where the side of the rectangle would be if SHIFT wasn't held down, while the blue line is the sqauare created by holding SHIFT.
Trick #2:
Place a mark where the top left and bottom left corners go. Now lightly draw a line 45°s; down-right from the top mark and a horizontal line from the bottom mark. The bottom right corner is where these two lines meet, and the top right corner is directly above it.