One-character variables like 'i' are acceptable when used as loop indices and in a limited number of other applications, but generally this is considered bad practice. Make sure your identifiers say something about what they represent. The same is true for procedure and function names. The use of vertical and horizontal white space goes a long way to making a program readable. Vertical white space refers to leaving blank lines within your code.
Indentation helps to indicate what control structure a section of code belongs to.
If you find you are indenting things more than say 15 spaces, you may be nesting things a little too deeply. See what you can do to avoid this, as it not only presents some readability problems, but also may cause a few problems with your logic. Beyond these guidelines, there really isn't anything you can do to improve white space. Please do not double-space your programs. Less than the minimal guidelines will earn you between 0 and 2 points.