Things that affect our applications' performance

[eluser]KeyStroke[/eluser]
Hello,

What are the things, that we should avoid in our code, that usually affect our applications' performace negatively?

I thought I'd ask this question here since there are so many experienced prfoessional programmers. I want to make high-performing applications, but have no idea what could hurt their performance (i.e: slow them down).

It would be very helpful if you could share with us some tips based on your experience.