Making your own resources is very important aspect of web design, that is often overlooked.
Why is it important? Simply because one of the other important aspects, (probably the most important one) is time management. No matter how great a designer, artist, or coder you are, time will be your constant enemy.

There will always be some element that was neglected, more testing that is required, and you simply won’t have the time. More importantly, why do the same task twice?  You can use proven code and other resources across multiple projects, and make minor variations as needed.

Bottom line, BUILD YOURSELF A RESOURCE LIBRARY!

Build a Modular CSS Framework
As a web designer you should already have a firm grasp of CSS (cascading style sheets) if you don’t other tutorials on this site and others can help you get the basics like developing table-less layout.  Most websites layouts have at least the following elements:

Header, footer, navigation, sub navigation, main content, sub content.

Knowing that you can build your own CSS Framework of proven code to reuse as you need it. The idea behind this is to build a small number of flexible documents as a references so that you won’t have to create your style sheets entirely from scratch and can manipulate them to work with a variety of content.

You should try to use as modular and approach as possible.

Building Graphics Resources
Elements you know you will need for nearly any web design project are, gradient backgrounds, buttons, dividers, banners, and occasionally media icons. Having the elements designed in advanced will ultimately save time even if they are not the final elements you choose to use because you can get an idea of what your completed design will be like rather than making assumptions on an abstract.

If you know you have to develop online banner ads and you haven’t already, you may consider building yourself Photoshop® presets to match the IAB’s (Internet Advertising Bureau) standards. It’s also not a bad idea to set up some Flash® preloaders that meet those requirements as well incase you need to do some time of interactive or animated banner for a client or yourself.

More to the point Photoshop® allows you to be very productive by building custom presets for you various tools like Brushes, Gradients, Shapes, Textures, Patterns and Abstract Backgrounds. The more of these you have, the faster you will be able to work by concentrating on arranging your elements and making precise tweaks here or there, rather than having to reproduce the same work over an over again from scratch.
A perfect example of this is creating a decent set of “Particle Brushes” and using them in your designs.

Set Your Code Up For Recycling
Every write a really cool script, form or application that you just know you could use again. Everyone has done this and then had to hunt through hundreds of files in their archives just to find it, only to remember that it won’t work with anything else.

Here are some helpful applications you should set up to be reusable anywhere, and tuck away for a rainy day. Or if you haven’t built anything to use these yet, you may as well do so, make them modular, and when you need them you will have them, and the ability to customize them to the project quickly and cleanly.

Login Panel, Email Contact Form, Polling Systems and Surveys, Shout Boxes, Content Management Systems, Image Galleries.

The Round Up
Doing a little extra work now, usually means not having to do a lot of it later, and in some cases never again. In your downtime if you organize and develop code, graphics, or even modular applications that you can use across multiple projects you will find that they never go to waste, and help you not waste time doing something more than once. As much as this is common knowledge, it is not “common practice”.