Never re-invent the wheel
Software Engineering is the process of developing practical and efficient solutions to problems using software. Good solutions should be problem free, seamless and deliver the customer the functionality they require.
In addition the best solutions should be well documented, easily maintained and able to be reused with other components.
The same basic design principles apply whether we talking about web design and web site scripts, software applications or databases. See our detailed pages on each of these design disciplines.