Throughout the course of my development career I have come across my own pearls of wisdom. I wish that someone would have been there when I started developing to tell me these things and it probably would have saved me a lot of time and work. On the other hand, maybe it was learning these things on my own that helped me be the best developer I can be.

1. If you write the same code more than once, you’re doing something wrong. Early on I can remember copying and pasting the same function into multiple files, then every time I had to change something in that function I had to update it in 20 different files. Today I still catch myself making notes in my code like “DUPLICATE FUNCTION EXISTS IN SUCH AND SUCH FILE”. Your code will be so much more efficient if you develop a way to never write the same code more than once.
2. If you have the same information stored in different database tables, you’re doing something wrong. If you are storing the same kind of information in multiple tables, then your database is inefficient. Some people do this to hopefully speed up their queries, but there are much better solutions to that problem.
3. If your database tables contain lots of null values, you’re doing something wrong. You should design your database in such a way that you never have any null values (unless they are unavoidable, which is hardly ever the case). Null values take up space and it’s inefficient to take up space with nothing.
4. If you have the exact same data stored in more than one place, you’re doing something wrong. This is also taking up space unnecessarily and is poor database design.
5. Learn how to code valid XHTML and do it. Standards aren’t standards unless everyone is trying to abide by them. With better standards in place, browsers can begin to be more compliant.

I hope these tips are useful to you.