<p>I am looking for easy steps that are simple and effective in making a web application more secure.</p>

<p>What are your top tips for secure web applications, and what kind of attack will they stop?</p>

Escape user provided content to avoid
XSS
attacks.
Using
paremeterised SQL
or stored procedures to avoid
SQL Injections
attacks.
Running the webserver as an unprivileged account to minimise attacks on the OS.
Setting the webserver directories to an unprivileged account, again, to minimise attacks on the OS.
Setting up unprivileged accounts on the SQL server and using them for the application to minimise attacks on the DB.
For more in depth information, there is always the
OWASP Guide to Building Secure Web Applications and Web Services