Keep an eye out for the red flashing arrows as these indicate where the next wave is coming from.
Make building a maze to keep enemies away from your base a priority and place cheap machine guns and goo towers on the perimeter for the early waves.
When the action starts to increase you can always pause the game at any time and completely reconfigure your defenses to better handle the onslaught. Remember though that doing this will mean you'll lose a bit of cash.
Upgrade your towers whenever you can, especially those on corners as it is a cheap way to add extra stopping power.